Body control module (BCM)
Definition
The body control module is the computer that manages the vehicle's non-powertrain electrics: interior and exterior lighting, central locking, windows, wipers, indicators, horn, alarm, key recognition and often the power distribution to other modules via internal relays and smart drivers. It reads switches directly or over LIN sub-buses and talks to the rest of the car over CAN, so a BCM fault can cause bizarre symptoms such as wipers that run when the indicator is used, lights that stay on, no crank because the start request is not forwarded, or a battery drain because the module never goes to sleep. Common causes are water ingress (blocked scuttle drains, leaking windscreen or door seals), corroded connectors, poor grounds, a short on a load that damages an output driver, and software bugs cured by an update. Many BCMs are coded to the vehicle (VIN, immobiliser, options), so a replacement must be programmed with manufacturer-level tools. Codes: U0140 (lost communication with BCM), B1xxx-B2xxx manufacturer body codes for lamps, switches and doors, U0155/U0164 (cluster and HVAC communication via BCM), P0513/P0633 (immobiliser key not programmed).