U0165 signals that the vehicle's network has detected a loss of communication from the rear HVAC control module. The PCM has identified that it is not receiving messages from this module.
The module's internal circuitry or temperature sensors can fail, causing it to stop transmitting messages on the CAN bus.
Corrosion, loose pins, or broken wires between the module and the vehicle network can interrupt communication.
If the fuse supplying power to the module opens, the module cannot send any signals.
A short to ground, voltage spike, or overall bus fault can cause multiple modules to appear offline, including the rear HVAC module.
PCM failure is uncommon; it would only be considered after all other possibilities are ruled out.
Replacing the PCM is rarely necessary; the issue is usually the rear HVAC module, its wiring, or a fuse.
Yes, a blown fuse that supplies power to the rear HVAC module can prevent it from communicating on the network.
No, only the specific rear HVAC control module or its wiring needs repair; the rest of the HVAC system can remain unchanged.
Diagnosis usually requires 1–2 hours to inspect wiring, check fuses, and test module communication.