The ABS-ECM continually monitors wheel speed from the signal provided by the wheel speed sensors.  If the ABS-ECM detects the incidence of wheel lock on one or more wheels, ABS is automatically initiated.

If the wheel speed sensor signals indicate that wheel rotation has now stabilised, the ABS-ECM will instigate the pressure build-up phase, allowing braking to continue.  If wheel lock is still detected after the pressure holding phase, the ABS-ECM instigates the pressure reduction phase.

If a fault is detected a fault code is stored in the ABS system.  A diagnostic tool is then used to erase the fault code as soon as repairs are carried out!