Check the ABS systems for any ABS error codes by bridging the ALDL connector pins 5 and 12.This will flash up ABS error codes on the ABS dashlight.Its a similar process to flashing up the ECU error codes on the engine dashlight.Heres a list of the different ABS error codes and what they mean.
Also inspect the toothed wheel behind the brake disc and the mounting of the sensor. There should be no damage to the teeth and the sensor should be securely mounted . It should be clean of the tiny magnetic rocks that tend to accumulate on the magnet in the sensor.
There is a spec on the gap between sensor and teeth but I don't know it off the top of my head. Check valve against the other (working) sensor on the LHS would be an easy first check.
Even a wheel bearing with excessive play in it can throw the signal off enough to get a fault code.
If it is the sensor you will need to buy a complete hub as the ABS sensor is part of the hub assembly, but first disconnect the battery to clear codes as well as checking cable to the sensor.

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