The ABS brake system performs several self-tests every time the ignition switch is turned ON and the vehicle is driven. The Anti-lock Brake Module (ABM) monitors the systems input and output circuits to verify the system is operating correctly. If the on board diagnostic system senses that a circuit is malfunctioning the system will set a trouble code in its memory. A scan tool must be used to diagnose the ABS system. Follow the prompts and testing procedures within the tool.