When the system "senses" impending lockup, braking pressure is automatically modulated in order to help prevent lockup that could lead to a skid.
The system performs a brief self-diagnostic test when the engine has been started and driver releases the brake pedal. Another automatic test may be performed when the vehicle first reaches a speed of approximately 6 mph (10 km/h). The brake pedal will pulsate several times and a sound may be audible from the ABS control module, which is normal.