Hi.
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 checking.
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.
Regards Slim.