Code B420, Twilight Light Relay Circuits: Notes

The BCM operates the twilight and courtesy light relays by providing a ground for the coils. When the relay(s) are on, the BCM output will be low, and when the relay(s) are off, the BCM output will be high. Code 420 will set if the ignition is on and the output voltage at the BCM is not what has been commanded.

NOTE: Test numbers refer to numbers on diagnostic chart.

To determine which system has failed (courtesy lights or twilight), select the BCM output function in diagnostic mode. As the individual outputs cycle on and off, observe their actual operation.