Brake pad wear sensors exist for one reason: to warn you before your pads wear out completely, so you replace them before metal touches rotor. There are two common types, and knowing which your car uses helps you understand the warning you get.
Mechanical (audible) wear indicators
The simplest type is a small spring-steel tab attached to the brake pad. As the friction material wears down to a set thickness, the tab starts to lightly contact the rotor and makes a high-pitched squeal - usually when you are NOT pressing the brake. That deliberate noise is your cue that the pads are near the end. It is cheap, effective, and needs no wiring. Learn more in our guide on why brakes squeak.
Electronic wear sensors
Many European and newer vehicles use an electronic sensor: a small wire loop embedded in the pad. When the pad wears far enough, the rotor grinds through the loop and breaks the circuit, which triggers a brake wear message or light on your dashboard. This gives you a clear dash warning instead of a noise.
Should you replace the sensor with the pads?
For electronic sensors, yes - if the sensor was worn through to trigger the warning, it is consumed and should be replaced along with the new pads. Many pad sets for these vehicles include new sensors, or they are sold alongside. For mechanical tab indicators, they are simply part of the new pad, so you get fresh ones automatically with a new pad set.
Do not ignore the warning
Whether it is a squeal or a dash light, a wear warning means it is time to inspect and replace the pads soon - before you damage the rotors. See our guide on when to replace brake pads, and add fresh hardware with every pad change.