One or more of the sensors, which are on each wheel next to the brakes, is/are not operating. Testing is by removing each one, and looking to see if it might be more worn than the others. Easy job if you can get under the car, one bolt 10/13mm they pull out towards you IE. same as the bolt comes out.
If that's not the cause of the light staying on, you've got a more serious problem with the ABS unit. Or it might just be a bad connection on the sensors. That's where I would start checking.
Ray Carcel, February 2008
