Citroen AX Fault Codes

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.
My 1994 Citroen AX had a running problem. The ECM was unable to read the Coolant Temperature Sensor (CTS) giving fault code 14. The CTS was replaced to no avail. The replacement died two years later. The sensor should read around 4k ohm at 10C, 2.2 - 2.8 k ohm at 20 C. It will go open circuit if the thermistor is dead. To change the CTS place a 19mm socket over the sensor before partially inserting a 1/2" drive. Placing the socket over the sensor with the drive fully inserted will break the sensor from its mounting.
Fault code 14 makes the ECM substitute a default value which makes the engine run badly. The problem was a poor connection on pin 25 of the multi-plug (P19.14 - the lead in the illustration is on the wrong end of the plug). The middle row starts at pin 20 from the lead end. (Check for continuity between pin 25 and the CTS first.) Removing and replacing the plug would cure the problem and remove the fault code temporarily. I eventually cured the problem by cutting the lead to pin 25 and making a permanent connection to the male pin on the ECM. This was done by cutting a 2 amp female spade connector and sliding it completely over pin 25. The lead was then soldered to the spade connector which gave a better joint than soldering directly to the pin. The lead should face away from the multi-plug so it is not crushed when the plug is replaced. I also had to connect one side of the CTS to the earth point on the suspension strut upper mounting, as both the earth and 5V signals were intermittent due to corrosion on the ECM multi-plug.
If the Throttle Potentiometer (TPS) wears out (mine did at 50,000), a replacement is around 300 quid (Citroen parts are around 25% cheaper in France). The manufacturing cost is probably no more than a pound. I wrote to the Consumers Association but they weren't interested. 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 ECM!
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