Brake pad wear sensor
Definition
The electrical pad wear sensor is a loop of wire embedded in a plastic body clipped to the pad (or a separate probe fitted to the caliper) that is ground through by the disc when the friction material reaches about 3 mm, opening the circuit and lighting the pad warning lamp; two-stage versions on BMW and others have two loops at different depths so the car can estimate remaining mileage. Because the warning is triggered by an open circuit, a chafed or broken wire, a corroded connector or a sensor pulled out of its clip also lights the lamp with pads that are fine, while a sensor incorrectly bridged or a cheap pad without a sensor slot never warns. Most systems fit a sensor on only one wheel per axle (usually left front and right rear), so the other wheel can be worn further without warning and must be inspected visually. Replace the sensor whenever it has been contacted by the disc, even if it still conducts; after fitting new pads and sensors on BMW, Mercedes and others, reset the service interval with a scan tool. Codes: VAG 01044/C1044 (brake pad wear sensor), BMW 5F3F/5F40 and 48xx-series, Mercedes B22xx brake wear circuit, Ford C1A1x; generic OBD has no standard code.