If one of the sensors was out of spec the light would stay on from is initial test period when you turn the ignition on. It will be a sensor not picking up properly I'm sure you can measure the ac voltage across the sensor when you spin the wheel. I can't remember of the top of my head what sort of result you should get though.
Do a search for posts containing abs by a bloke called jon@work I'm sure he had posted up some good technical info.