So, tonight you feel confident driving home through several inches of freshly fallen snow after a sumptuous holiday dinner. Your front-drive car has excellent all-season tires and ABS (antilock braking system)—although the ABS light has been on since you banzai’d the berm at the end of the driveway an hour ago. This may explain the loss of steering control when you’re slowing down for a corner. Like this downhill turn, right … there, as you blow straight past it with the wheels skidding and the steering cranked over hard against the stop.

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.