Code B420 - Twilight Relays Circuit: Notes

If headlights and parklights did not cycle on and off when BO05 was selected, then twilight relay set code B420.

1. Checks if fault is caused by an open circuit or a short to ground. If headlights and parklights are always on, then there is a short to ground in control circuit. If headlights are never on, then there is an open in one of the circuits.
2. Checks if short to ground is due to circuit or BCM.
3. Checks if open circuit is due to circuit or BCM.
4. Checks if open circuit is on power or ground side of relay.