BCM Code B420: Twilight Relays Circuit: Notes
WARNING: This page is about a different car, the 1990 Cadillac Seville and 1990 Cadillac Eldorado. However, it is still accessible from the selected car via links, so may be relevant.
If headlights and parklights did not cycle on and off when BO05 was selected, then twilight relay Code B420 is set.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
Test Number 1) Check to see 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.
Test Number 2) Check to see if short to ground is due to circuit or BCM.
Test Number 3) Check to see if open circuit is due to circuit or BCM.
Test Number 4) Check to see if open circuit is on power or ground side of relay.