The ABS (Anti-lock Braking System) sensor, also known as a wheel speed sensor, is located at the wheel hub, near the brake rotor. It uses a magnet or Hall effect sensor to read the teeth on a tone ring as the wheel spins. It constantly compares the speed of each wheel; if one wheel slows down significantly faster than the others during braking (indicating a skid), the system pulses the brakes. These sensors are exposed to extreme heat, brake dust, and road debris, making them a common failure point that triggers the ABS warning light.