auto-sensor

Brake pad wear sensor, also known as the brake pad wear indicator. The most common type of electronic wear sensors are mounted into the brake pad itself. On some applications the wear sensor is embedded into the pad and not removable; therefore they are only available with the brake pad. When the brake pad wears down to its discard thickness due to contacting the rotor while braking over the course of time, the sensor will make contact to the rotor, thus grounding the sensor.

This will cause a brake warning light to illuminate on your dash or instrument cluster, indicating that the brake pad friction material is almost gone and that replacement brake pads are needed. This light usually looks like a circle, sometimes with an exclamation mark inside of it.