Common in higher end or luxury vehicles, the disk brake pad wear sensor is designed to alert drivers when the brake pad is approaching the end of its life and needs to be replaced. The sensor is very simple, effectively a loop of wire that extends up from the back of the brake pad to the minimum spec depth. As long as the electrical loop is complete, the light will stay off. As soon as the brake pad wears to the minimum thickness, the loop will be exposed to the brake rotor and will break. As soon as the wire breaks, the light will illuminate.

If your vehicle comes with wear sensors, it is important to buy new ones with every brake job to avoid having a continuous light illuminated on your dash.