The light will go off when the fault is fixed.
Possibly the sensor has been damaged or knocked out of alignment, or the toothed wheel (reluctor ring) has been cracked or chipped.
Make sure the ring and the face of the sensor are clean, any metal or rust sticking to the sensor can confuse it.
Make sure the sensor wiring is ok.
You can do a basic test of the sensor by measuring the resistance using a multimeter.
I dont know off-hand what it should read, compare it to the others.
When you spin the wheel the resistance should jump up and down.
vic, July 2009