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
Page 19.16 Fault code table (Citroen and Peugeot)
11 End of diagnosis
12 Initiation of diagnosis
22 Stepper motor
31x Lambda control
42 Injector or fuel pump control
52 Lambda control
53x Battery voltage
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
long lengths of cable). To clear the fault codes short for 10
or remove the ECM multi-plug. The FCR is situated by the o/s
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
memory of the ECU!