ABS sensors measure wheel speed and relay the info to the ECU. If you hit the brakes hard and lockup the wheels the ECU will detect that the road speed doesn't match the wheel speed and will release the brake on the locked wheel. The ABS system will release and re-apply the brakes right up to the point of locking, multiple times a second. If you've ever slammed on your brakes and felt the pedal push back, combined with a strange vibrating or buzzing noise, that's your anti lock braking system kicking in. The ABS sensors are also used by the vehicles stability control system and help keep you safe in the event of a skid or slide. So you can appreciate the importannt role ABS sensors play in vehicle safety. Generally, they're not expensive to replace when they do fail and it's one part that's not worth scrimping on.