THEY ARE NOT REALLY SENSORS IN THE TRUE TERM AS IN ELECTRICAL SENSOR , BUT THEY ARE SQUEELERS ON EACH INSIDE BRAKE PAD THAT RUBS AGAINST THE ROTOR WHEN THE PADS WEAR OUT, THEY ARE PART OF THE METAL SHIM THAT GOES IN BEFORE THE PADS GO ON ,USUALLY WHICH ARE LEFT OUT ON AFTERMARKET PADS , SAME DESIGN AS ON THE REAR JERRY