Code B420, Relay Circuits
BCM operates twilight, courtesy light and fog light relays by providing a ground for relay coils. When relay(s) are on, BCM output will be low and when relay(s) are off, BCM output will be high. BCM also provides an alternating high/low clock signal to Driver Information System (DIS) switch pod to facilitate communications.
Code B420 will be set if ignition is on and output voltage at BCM is not what has been commanded.
NOTE:
Test numbers refer to numbers on diagnostic chart.
Test Number 1) To determine if courtesy light system has failed.
Test Number 2) To determine if twilight system has failed.
Test Number 3) To determine if fog light system has failed.
Test Number 4) To determine if Driver Information System clock signal has failed.