To check from the ECU harness.
Switch off ignition
Remove ABS ECU wiring plug from the ECU
Using an ohm meter check the terminals on the connector, not the ECU
Terminals 1 and 6 = Right Front
Terminals 15 and 30 = Left Front
Terminals 29 and 31 = Right Rear
Terminals 19 and 28 = Left Rear
Just in case !! The left and right is refered to as from the driving
position, ie from the rear of the car, not as you will be standing and
Also, the thickness of the discs and pads will make no diference at all
to the ABS as the sensor is fixed into the hub and the pick up ring is
on the drive-shaft. The gap (air gap) is set on the Xantia and cannot
be moved, so I doubt if that will be your trouble.
When fitting a new sensor, the ABS light may not go out as soon as
started. Citroen will tell you that it needs to be fitted by them and
then reset on the computor! This aint true, all you have to do is put
the wheel on and drive, the thing will reset as soon as you reach
approx 15 mph
Hope this helps in some way, let us know how you get on.