The ABS sensor (wheel speed sensor) monitors the rotation speed of each tire. Mounted on the steering knuckle or axle housing, the sensor tip sits close to a toothed ring that spins with the wheel. As the teeth pass the magnetic sensor, it generates an electrical frequency signal. The ABS computer reads this signal to detect if a wheel is locking up during braking. It also supplies data to traction control and stability control systems.
These sensors live in a harsh environment. They are exposed to road water, salt, and brake dust. Rust can push the sensor tip away from the ring, or iron filings can stick to the magnetic tip, distorting the signal. If the sensor fails, it triggers the ABS warning light and disables the system, leaving you with standard power brakes.