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 B123 - Courtesy Light Switch: Notes
The courtesy lights panel switch is part of the dimmer control potentiometer. In left switch assembly the "COURTESY LIGHTS ENABLE" switch closes when dimmer is moved to extreme right. The BCM supplies signal voltage on circuit No. 685 which is pulled to ground through circuit No. 736 when switch contact is closed.
Code B123 will set when ignition is on, dimming potentiometer is working properly (code B122) and signal voltage is high when dimmer is moved to enable position or signal voltage is low when dimmer is moved to maximum dim position.
- BCM input value B101 displays voltage state of circuit No. 685 at BCM.
- Checks if open circuit reading (HI) is caused by circuit or switch assembly. If display changes from "HI" to "LO" when switch terminals are jumped together, BCM and wiring are okay.
- Checks if open circuit reading is caused by BCM or wiring.
- With courtesy lights switch off, checks if short to ground reading is due to switch assembly or wiring.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
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.