Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Code B420 - Relay Circuits: Notes
The BCM operates twilight, retained accessory power, high/low beam and courtesy light relays by providing a ground for the coils. When relay or relays are on, BCM output will be low and when relays are off, BCM output will be high. Code B420 will set if ignition is on and output voltage at BCM is low when a high has been commanded.
- To determine which system has failed, select BCM output function in diagnostics. As individual outputs cycle on and off, observe their actual operation.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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