Code B420, Twilight Light Relay Circuits (TORONADO): Notes

Fig 1: Code B420, Wiring Diagram Twilight Light Relay Circuits (Toronado)

If headlights and parking lights did not cycle on and off when BO05 was selected, this is the system that set Code B420.

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