Code B420 (C), Fog Light Relay Circuit: Notes

If fog lights did not cycle on and off when BO12 was selected, then this is system that set code B420.

Test Number 1)  Checks to see if fault is due to an open circuit or a short to ground in circuit. If fog lights are always on, then there is a short to ground in control circuit and if fog lights never turn on, then there is an open in one of the circuits.

Test Number 2)  Isolates BCM from circuit problems.

Test Number 3)  Checks to see if open circuit is on power or ground side of relay.