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!
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!