Citroen AX Fault Codes

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But they won’t sell you the potentiometer as a separate part!
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

Citroen AX Fault Codes
I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug could have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi-plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi-plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But they won’t sell you the potentiometer as a separate part!
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

Citroen AX Fault Codes
I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But they won’t sell you the potentiometer as a separate part!
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor Citroen Xantia Lancia 3437022” from Berlin at around £30 all in! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted.
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. If a multimeter is available, the resistance between pins 1 and 2 (rearmost pair) should be set to around 1070 ohms.
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. If a multimeter is available, the resistance between pins 1 and 4 should be set to around 940 ohms (pins are numbered 1, 2, 4, 5 from back to front).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Silicone grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Silicone grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed.
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). If necessary the slotted holes in the potentiometer can be elongated with a small file. Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed, or disengaged by applying 4.5V to the top 2 pins (positive to top pin). Remember to reengage the stepper motor by reversing the polarity and reconnect the plug or the customer’s car will keep stalling until the engine warms up!
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344) Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the memory of the ECU!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). If necessary the slotted holes in the potentiometer can be elongated with a small file. Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed, or disengaged by applying 4.5V to the top 2 pins (positive to top pin). Remember to reengage the stepper motor by reversing the polarity and reconnect the plug or the customer’s car will keep stalling until the engine warms up!
The original ECU is a Bosch MA3.0 no. 0 261 200 780 (Peugeot 106 1 litre 93 on). I have owned three of these and found them all unreliable, giving false fault codes. I was advised by Bosch that this model has been superceded by MA3.0 No. 0 261 204 051 (Peugeot 106 1 litre 95-96). This model works reliably on my car, although it gives a 27 fault code (VSS) as there is no vehicle speed sensor fitted to the 954cc Citroen AX (there is no contact on pin 9 of the ECU multi-plug).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344)
Page 19.14 Pin table - typical 55-pin (Citroen/Peugeot)
1 Ignition coil        13 SD connector    27 ATS 2 Earth            14 Earth        28 OS 3 Engine control module relay    15 ISCV        29 TPS 4 –            16 SD connector    30 RPM sensor 5 CFSV            17 Injector        31 ISCV 6 Tachometer        18 Battery supply    32 - 7 TPS            19 Earth        33 ISCV 8 –            20 Ignition coil    34 - 9 VSS            21 -        35 - 10 OS            22 SD warning lamp    36 - 11 RPM sensor        25 CTS        37 Engine control module relay 12 TPS            26 Sensor return (CTS, ATS, TPS)
Typical 55-pin multi-plug (Citroen and Peugeot)
19 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _1 (cable end) 37 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _20 55 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _38
Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes on the original ECU. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the ECU memory!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). If necessary the slotted holes in the potentiometer can be elongated with a small file. Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed, or disengaged by applying 4.5V to the top 2 pins (positive to top pin). Remember to reengage the stepper motor by reversing the polarity and reconnect the plug or the customer’s car will keep stalling until the engine warms up!
The original ECU is a Bosch MA3.0 no. 0 261 200 780 (Peugeot 106 1 litre 93 on). I have owned three of these and found them all unreliable, giving false fault codes. I was advised by Bosch that this model has been superceded by MA3.0 No. 0 261 204 051 (Peugeot 106 1 litre 95-96). This model works reliably on my car, although it gives a 27 fault code (VSS) as there is no vehicle speed sensor fitted to the 954cc Citroen AX (there is no contact on pin 9 of the ECU multi-plug).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344)
Page 19.14 Pin table - typical 55-pin (Citroen/Peugeot)
1 Ignition coil 2 Earth 3 Engine control module relay 4 – 5 CFSV 6 Tachometer 7 TPS 8 – 9 VSS 10 OS 11 RPM sensor 12 TPS 13 SD connector 14 Earth 15 ISCV 16 SD connector 17 Injector 18 Battery supply 19 Earth 20 Ignition coil 21 – 22 SD warning lamp 25 CTS 26 Sensor return (CTS, ATS, TPS) 27 ATS 28 OS 29 TPS 30 RPM sensor 31 ISCV 32 – 33 ISCV 34 – 35 – 36 – 37 Engine control module relay
Typical 55-pin multi-plug (Citroen and Peugeot)
19 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _1 (cable end) 37 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _20 55 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _38
Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes on the original ECU. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the ECU memory!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). If necessary the slotted holes in the potentiometer can be elongated with a small file. Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed, or disengaged by applying 4.5V to the top 2 pins (positive to top pin). Remember to reengage the stepper motor by reversing the polarity and reconnect the plug or the customer’s car will keep stalling until the engine warms up!
The original ECU is a Bosch MA3.0 no. 0 261 200 780 (Peugeot 106 1 litre 93 on). I have owned three of these and found them all unreliable, giving false fault codes. I was advised by Bosch that this model has been superceded by MA3.0 No. 0 261 204 051 (Peugeot 106 1 litre 95-96). This model works reliably on my car, although it gives a 27 fault code (VSS) as there is no vehicle speed sensor fitted to the 954cc Citroen AX (there is no contact on pin 9 of the ECU multi-plug).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344)
Page 19.14 Pin table - typical 55-pin (Citroen/Peugeot)
1 Ignition coil 2 Earth 3 Engine control module relay 4 – 5 CFSV 6 Tachometer 7 TPS 8 – 9 VSS 10 OS 11 RPM sensor 12 TPS 13 SD connector 14 Earth 15 ISCV 16 SD connector 17 Injector 18 Battery supply 19 Earth 20 Ignition coil 21 – 22 SD warning lamp 25 CTS 26 Sensor return (CTS, ATS, TPS) 27 ATS 28 OS 29 TPS 30 RPM sensor 31 ISCV 32 – 33 ISCV 34 – 35 – 36 – 37 Engine control module relay
Typical 55-pin multi-plug (Citroen and Peugeot)
19 - - - - - - - - - - - - - - - - - - - 1 (cable end) 37 - - - - - - - - - - - - - - - - - - 20 55 - - - - - - - - - - - - - - - - - - 38
Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes on the original ECU. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the ECU memory!
Happy motoring!

I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi- plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi- plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes!
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). If necessary the slotted holes in the potentiometer can be elongated with a small file. Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed, or disengaged by applying 4.5V to the top 2 pins (positive to top pin). Remember to reengage the stepper motor by reversing the polarity and reconnect the plug or the customer’s car will keep stalling until the engine warms up!
The original ECU is a Bosch MA3.0 no. 0 261 200 780 (Peugeot 106 1 litre 93 on). I was advised by Bosch that this model has been superceded by MA3.0 No. 0 261 204 051 (Peugeot 106 1 litre 95-96). This model gives a 27 fault code (VSS) as there is no vehicle speed sensor fitted to the 954cc Citroen AX (there is no contact on pin 9 of the ECU multi-plug).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344)
Page 19.14 Pin table - typical 55-pin (Citroen/Peugeot)
1 Ignition coil 2 Earth 3 Engine control module relay 4 – 5 CFSV 6 Tachometer 7 TPS 8 – 9 VSS 10 OS 11 RPM sensor 12 TPS 13 SD connector 14 Earth 15 ISCV 16 SD connector 17 Injector 18 Battery supply 19 Earth 20 Ignition coil 21 – 22 SD warning lamp 25 CTS 26 Sensor return (CTS, ATS, TPS) 27 ATS 28 OS 29 TPS 30 RPM sensor 31 ISCV 32 – 33 ISCV 34 – 35 – 36 – 37 Engine control module relay
Typical 55-pin multi-plug (Citroen and Peugeot)
19 - - - - - - - - - - - - - - - - - - - 1 (cable end) 37 - - - - - - - - - - - - - - - - - - 20 55 - - - - - - - - - - - - - - - - - - 38
Page 19.16 Fault code table (Citroen and Peugeot) Code Item 11 End of diagnosis 12 Initiation of diagnosis 13x ATS 14x CTS 21x TPS 22 Stepper motor 27x VSS 31x Lambda control 41 CAS 42 Injector or fuel pump control 51 OS 52 Lambda control 53x Battery voltage 54 ECM x Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes on the original ECU. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the ECU memory!
Happy motoring!

Citroen AX Fault Codes
I own a 1994 Citroen AX, 954cc fitted with single-point petrol injection controlled by a Bosch Mono-Motronic MA3.0 Electronic Control Unit/Module (ECU or ECM). It recently began to run badly, particularly when driving at speed on long journeys. The ECU kept giving fault code 51, indicating a faulty Oxygen Sensor (Lambda Sensor). It was replaced with a universal 4-wire zirconia sensor from eBay, but the problem persisted. This was then replaced with the correct Bosch sensor, only for the problem to reoccur. The exhaust was checked for leaks and the bolts tightened on the exhaust manifold, to no avail. The throttle body was then removed and Hylomar silicone gasketing compound applied to both sides of the top and bottom gaskets, again without success. Finally the inlet manifold was examined, which appeared sound. However it was removed (requiring the radiator fluid to be drained and the throttle body to be removed again), the old rubber cement rubbed off and replaced with Hylomar on both surfaces (it’s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my amazement, the problem vanished! (Rubber based compounds may be prone to perishing over long periods and high temperatures, while silicone is not.)
The ECU now gives fault code 14 only, causing the engine to run badly, indicating a faulty Coolant Temperature Sensor (Citroen Part No. 24246 – £22). A new sensor (thermistor) still generates the same fault code and the wiring is okay. It could be an ECU fault, although it occurs on two other ECUs. Bosch charge £360 for a replacement unit and there is no documentation available! So I suspected that the multi-plug may have a bad contact – computer PCBs have had gold plated contacts for years but the motor industry doesn’t like reliable cars! To overcome this problem, wires have to be soldered onto the internal contacts of the multi-plug socket. First remove the ECUs plastic cover, the multi-plug and the four retaining 8mm bolts. Once indoors, remove the five T15 torx screws. The top plate may need levering off as it is sealed with silicone sealant. Two thin insulated wires are soldered onto pins 25 and 26 (CTS and sensor return – polarity not important). (Pin 20 is on top at the cable end but do a continuity check between the multi-plug pins and the sensor connector to be sure you have pins 25 and 26.) It should be possible to close the top plate over the thin insulated wires without damage. The lead to the sensor plug is cut and a new cable soldered in and sealed at either end with heat shrink tubing - the original wiring loom is inaccessible so the bare ends are also covered with heat shrink insulation, which is more durable than tape. The new cable should be routed behind the block alongside the wiring loom. No more 14 fault codes! (Alternatively a new 55-pin multi-plug can be fitted with soldered pins, although this is time consuming.)
The other problem on the Citroen AX is the throttle potentiometer which wears out (fault code 21), and is only available with the throttle body (Citroen Part No. 1920X4) at around £300! Bosch do an exchange unit for around £170 (Bosch Part No. 0 986 438 671) + £3 odd for a 5 pack of gaskets (Bosch Part No. 3 431 015 900). But Bosch won’t sell you the potentiometer as a separate part. Happily the potentiometers have now appeared on eBay as “Throttle Position Sensor for Citroen, Peugeot 3437022” priced at £20! The position of the original potentiometer should be marked before removing the four tamper proof T20 torx screws, as the fixing holes are slotted. IMPORTANT: Teflon grease should be applied to the tracks and wipers to prevent wear – Bosch don’t use any lubricant! If a multimeter is available, the resistance between pins 1 and 2 should be set to around 1070 ohms (pins are numbered 1, 2, 4, 5 from back to front). If necessary the slotted holes in the potentiometer can be elongated with a small file. The four wipers may need bending upwards as the pattern part is slimmer than the original Bosch. Ensure that the measurement is made with the throttle closed – the stepper motor opens the throttle when the engine is cold and can be temporarily removed, or disengaged by applying 4.5V to the top 2 pins (positive to top pin) – a 9V PP3 battery can be used without damage. Remember to reengage the stepper motor by reversing the polarity and reconnect the plug or the customer’s car will keep stalling until the engine warms up!
The original ECU is a Bosch MA3.0 no. 0 261 200 780 (Peugeot 106 1 litre 93 on). I was advised by Bosch that this model has been superceded by MA3.0 No. 0 261 204 051 (Peugeot 106 1 litre 95-96). This model gives a 27 fault code (VSS) as there is no vehicle speed sensor fitted to the 954cc Citroen AX (there is no contact on pin 9 of the ECU multi-plug).
From Haynes Automotive Engine Management and Fuel Injection Systems Manual (3344)
Page 19.14 Pin table - typical 55-pin (Citroen/Peugeot) 1 Ignition coil 2 Earth 3 Engine control module relay 4 – 5 CFSV 6 Tachometer 7 TPS 8 – 9 VSS 10 OS 11 RPM sensor 12 TPS 13 SD connector 14 Earth 15 ISCV 16 SD connector 17 Injector 18 Battery supply 19 Earth 20 Ignition coil 21 – 22 SD warning lamp     25 CTS 26 Sensor return (CTS, ATS, TPS) 27 ATS 28 OS 29 TPS 30 RPM sensor 31 ISCV 32 – 33 ISCV 34 – 35 – 36 – 37 Engine control module relay
Typical 55-pin multi-plug (Citroen and Peugeot)
19 - - - - - - - - - - - - - - - - - - - 1 (cable end) 37 - - - - - - - - - - - - - - - - - - 20 55 - - - - - - - - - - - - - - - - - - 38
Page 19.16 Fault code table (Citroen and Peugeot)
Code    Item 11    End of diagnosis 12    Initiation of diagnosis 13x    ATS 14x    CTS 21x    TPS 22    Stepper motor 27x    VSS 31x    Lambda control 41    CAS 42    Injector or fuel pump control 51    OS 52    Lambda control 53x    Battery voltage 54    ECM X    Faults that typically will cause the ECM (Electronic Control Module) to enter LOS (limited operating strategy) and use a default value in place of the sensor.
To obtain fault codes short the green lead side of the Fault Code Reader (FCR) multi-plug to earth for 4 seconds with the ignition on while an assistant watches the fault lamp on the dashboard (or use two long lengths of cable). To clear the fault codes short for 10 seconds, or remove the ECM multi-plug. Fault code 21 cannot be removed if the throttle potentiometer resistance is wrong and this fault can generate other fault codes on the original ECU. The FCR is situated by the o/s headlamp. If you're buying second hand check the fault codes. Take the vehicle for a run first in case the owner has cleared the fault codes from the ECU memory!
Happy motoring!

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Thought you had died! You have not posted this for six months now.

known as Trevor, that requests spam to be send to snipped-for-privacy@nothere.com and I became inspired.
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Citnutters never die.

  Quoted Text Here  
But some of us rattle a bit sometimes.

made by the entity known as Whiskers, that requests spam to be send to snipped-for-privacy@operamail.com and I became inspired.
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That is only for the very very old.... :)