My 1994 Citroen AX recently began to run badly, particularly when driving at speed on long journeys. The ECM/ECU kept giving fault code
51, indicating a faulty Oxygen Sensor (Lambda Sensor). I replaced it with a universal 4-wire zirconia sensor from eBay, but the problem persisted. I then replaced this with the correct Bosch sensor, only for the problem to reoccur. I checked the exhaust for leaks and tightened the bolts on the exhaust manifold, to no avail. I then removed the throttle body and applied Hylomar silicone gasketing compound to both sides of the top and bottom gaskets, again without success. Finally I examined the inlet manifold which appeared sound. However I removed it (requiring the radiator fluid to be drained and the throttle body to be removed again), rubbed the old rubber cement off and replaced it with Hylomar on both surfaces (it?s easier to apply a thin coat with a finger rather than with the nozzle supplied). To my surprise the problem vanished! I can only speculate that rubber based compounds are prone to perishing over long periods and high temperatures, which silicone is not.The other problem on the Citroen AX is the throttle potentiometer which wears out, 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!