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.
Body Control Modules: Description
The Body Control Module (BCM) communicates with various systems over the serial data line in order to control the following functions:
- Bulb over-voltage protection.
- Door, hatch and hood ajar status.
- Exterior and interior lighting control.
- Hatch release control.
- Heated mirror control.
- Horn relay.
- Key-in-ignition status.
- Load shed control.
- Monitored loads control.
- PASS-Key(R) system.
- Power mode master.
- Rear defogger control.
- Steering column lock/unlock control.
- Traction Control System (TCS) activation control.
- Universal Theft Deterrent (UTD).
- Vehicle configuration (RPO codes).
Some vehicle systems share data over a serial data line to execute various vehicle functions. Communication between each system is accomplished by sending digitally coded messages, which consist of specific information the system module must follow. Each system module is assigned its own recognition code, so that it can respond to appropriate messages. The BCM acts as the power mode master and sends power mode messages on the serial data line. Power mode messages are ignition switch position. Systems not directly communicating with the ignition switch use the power mode messages to determine ignition switch position. Systems other than the BCM respond to the power mode messages with a State Of Health (SOH) code message. The SOH messages lets the BCM know that those systems can communicate with other systems.
Signals that activate the BCM are known as wake-up signals. BCM is asleep when it is not controlling or monitoring the following wake-up functions:
- Any activity on serial data line.
- Hatch or door ajar switch.
- Hatch release switch.
- Key-in-ignition switch.
- Monitored load relay.
- Any exterior lighting function.
- Battery disconnection and reconnection.
- Ignition is turned on.
The BCM will enter a sleep state when there is no activity on serial data line, ignition is turned off, no outputs are commanded, no delay timers are actively counting and no wake-up inputs are present.
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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.