Anti-lock brake system wheel speed sensors monitor wheel speed and send an analog or digital signal to the ECM to help prevent the brakes from locking during a stop. The ECM uses this signal to determine the amount of wheel lock-up and/or wheel slippage for the anti-lock brake, traction control and stability control systems. Causes for replacement include ABS/brake lamp illumination, no or erratic ABS, traction control or stability control operation.