Brake sensor wires should be replaced at the same time the brake pads are replaced whether or not the brake pad warning light has come on. While in the brake pad, the brake wear sensors are subjected to a lot of heat which makes the insulation hard and susceptible to cracking. If the insulation cracks, brake dust can collect and cause the pad warning light to come on unnecessarily by making contact with the wires in the sensors. The sensor merely provides a ground to the warning light on the dash when the rotor wars away the insulation on the sensor.