Lost Communication With Rear HVAC Control Module
Your vehicle's main computer has lost the ability to talk to the air conditioning control module in the back seat, like a phone call dropping mid-conversation. The rear climate control system can't receive commands or send status updates to the vehicle's network.
The main ECU monitors CAN bus communication signals from the rear HVAC module at regular intervals. When no valid message frames are received within the timeout period, the ECU logs this as lost communication. The system expects periodic status and acknowledgment messages on the network.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| CAN Bus Message Timeout | Message received every 100-500ms | No message received for >1 second |
| Signal Integrity | Valid CRC and checksum confirmed | Corrupted or missing frames detected |
Code U0165 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, U0165 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.