ABS has become pretty much standard equipment on most vehicles. Sensors tell a computer when a wheel stops rotating, which indicates—at least when the vehicle still has forward speed—that the brakes have overpowered the available traction at that particular wheel. The computer then directs a hydraulic valve to release some brake fluid pressure to the wheel to let it rotate again.

This process repeats many times per second until the vehicle stops or you lift your foot off the brake pedal. The ABS computer does a power-on self-test every time you cycle the ignition. If it finds it’s lacking data, or a hydraulic pump or valve isn’t responding, it illuminates the ABS warning light on the dash.