Code B420: Courtesy Lamp Relay Circuits: Notes
WARNING: This page is about a different car, the 1990 Buick Riviera and 1990 Buick Reatta. However, it is still accessible from the selected car via links, so may be relevant.
If courtesy lamps did not cycle on and off when BO04 display was selected, this is the relay that set Code B420.
NOTE:
Test numbers refer to numbers on diagnostic chart.
Test Number 1) Checks to see if fault is due to an open circuit or a short to ground in circuit. If courtesy lamps are always on, there is a short to ground in control circuit and if courtesy lamps never turn on, there is an open in one of the circuits.
Test Number 2) Checks to see if short to ground is due to circuit or BCM.
Test Number 3) Checks to see if open circuit is due to circuit or relay.
Test Number 4) Checks to see if open circuit is on power or ground side of circuit.
Test Number 5) Checks to see if open circuit is in wiring or BCM.