When the brake pad wears and the Electronic Brake wear sensor makes contact with the rotor a warning is given usually in the form of a light on the vehicles dashboard. 

Wear sensors are designed to break or complete a circuit as the brake pad wears and hence are not re-useable and need to be changed whenever the brake pads are replaced or the warning light indicates.

Features:

• Electronic Brake Wear Sensors should be changed whenever the pads are replaced
• If they have made contact with the rotor they cannot be reused
• Even if they have not made contact they should still be replaced with every pad change
• Materials become worn during their service life and can affect their functionality
• To ensure performance throughout the life of the pads they should be replaced with a new sensor