Code B420 (A), Courtesy Lamp Relay Circuits

BCM operates courtesy lamp relay by providing ground for relay coil. When relay is on, BCM output will be low and when relay is off, BCM output will be high.

Code B420 will set if ignition is on and output voltage at BCM is not what has been commanded.

Test Number 1)  Determines which way system has failed.

Test Number 2)  Checks to see if short or ground is due to circuit or BCM.

Test Number 3)  Verifies relay signal from BCM.

Test Number 4)  Checks to see if open circuit is on power or ground side of relay.

Test Number 5)  Checks to see if open circuit is on ground side of relay or in relay.

Fig 1: Code B420 (A): Circuit Diagram

Fig 2: Code B420 (A): Flow Chart Courtesy Lamp Relay Circuit