This is from Jim Levie
If the light goes out after engine start and only comes on when the car is
moving, there’s a problem with one (or more) of the sensors. The simple test
to find out which sensor(s) is the cause is to disconnect all but one of the
sensors and drive the car. If the light comes on, that sensor is sending a
speed signal to the ABS and is good. Repeat with each other sensor to find the
one(s) that don’t cause the light to come on. Since the ABS module has no
speed input besides the wheel sensors, if only a bad sensor is connected the
module can’t tell the car is moving and thus doesn’t fault on missing speed
signals from the other wheels. Once a bad speed sensor is found, if a new
sensor still doesn’t work, there could be a wiring fault in that circuit or a
bad input channel in the ABS unit.