Lost Communication With Body Control Module
Your engine's main computer lost its connection to the body control module, which handles things like lights, windows, and doors. It's like two walkie-talkies that stopped communicating with each other.
The ECM communicates with the BCM via CAN bus protocol, monitoring for regular acknowledgment messages. If no valid signal is received within a specified timeout window (typically 100-500ms), the ECM logs a communication fault and sets the DTC.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| CAN Bus Message Timeout | BCM responds within 100-500ms | No BCM response or timeout exceeded |
| CAN Bus Signal Voltage | 2.5-3.5V differential | Below 0.5V or above 5V |
Code U0140 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, U0140 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.