U0140

What the Code Means

U0140 logs a loss of communication with the body control module (BCM). This module handles body functions: lighting, locks, and window regulators. The BCM exchanges data with the other modules over the CAN bus, and when another module stops getting replies from it, the DME stores this code.

There are three main causes for the lost communication: a faulty BCM itself, damaged wiring, or a problem on the CAN bus — an open circuit, a short, or a missing terminating resistor.

Symptoms

• Check Engine light is on.
• Interior or exterior lighting faults.
• Window regulators or locks don't work.
• The diagnostic tool can't establish communication with the BCM.

Diagnosis and Repair

1. Check BCM power and ground: a missing one is a common cause of a “dead” module that looks faulty.
2. Check the CAN bus wiring for opens, shorts, and damaged insulation.
3. Check the BCM connectors — corroded contacts or a connector that isn't fully seated produce the same symptoms as a failed module.
4. Run diagnostics through ISTA: a scan of all modules shows which ones lose communication and whether it drops completely or intermittently.
5. With a confirmed BCM failure, replace the module and program it. Without programming, the new module won't work on the vehicle network.

Secondary Problems

When the BCM fails, most comfort functions stop working: lighting, locks, window regulators. If the immobilizer is integrated into the BCM, the car may not start.

Putting off diagnosis is critical: the code is flagged as critical, and the repair is complicated by the fact that replacing the BCM requires programming, which makes the replacement option expensive.