Brake pads are wearing constantly due to the friction and heat produced by the braking process. When the pad wears down to a predetermined level, the brake pad sensor within comes into contact with the metal brake disc and creates an electrical circuit. This in turn illuminates a warning light on the dash board which informs the driver that the brakes need replacing.
Correctly functioning brake pad sensors for every make and model of car are vital, as without them it may not be readily apparent that the brakes need replacing. This is as important for road safety as it is for NCT purposes.