Brake Pad Wear Indicators
Pad wear indicators have evolved from the standard metal clip or audible sensor to the electronic wear sensor. They both perform the same basic function of warning the driver when the brake pads have worn down enough to require replacement. 
The audible clips have been around for along time and are still in use on many late model vehicles today. They can come affixed to the brake pad or may have to be removed and placed onto the new brake pad before installation. These clips come into contact with the brake rotor when the pad wears down to a certain thickness. This contact produces a loud squeal when the clip touches the rotor warning the driver of an impeding brake problem.
Electronic sensors provide a warning light for the driver as well as a less intrusive way of indicating pad wear. When this wire circuit is interrupted, either completing or breaking the circuit, the pad wear warning light will be activated on the instrument panel. The down side to all this is by the time the drivers light comes on the sensors circuit has already been broken often requiring sensor replacement. These sensors work great, can be tested with an ohmmeter, and are usually found on the high-end models.
