The ABS or Anti-Lock Brake System ECU monitors the Wheel Speed Sensors rotational speed for anti-lock brake function, stability control and traction control. The ABS sensors report the speed of all four wheels to the ECU to determine if the wheels are locking up and to help provide safer stopping. Reasons for replacement include: ABS/brake lamp illumination, no or erratic ABS operation or problems with stability and or traction control systems.