What it means
The vehicle bus reports loss of communication with smart rear HVAC mode (vent/floor/defrost) actuator. Rear air distribution falls back to a default mode.
How it works
The rear mode actuator is a smart HVAC component that moves doors to direct rear air through the selected outlets, such as vents or floor ducts. U0663 sets when network messages from this actuator are lost, so rear airflow distribution may remain at or revert to a default mode.
Common causes
Most likely first.
- Bus wiring open or shorted
- Module without power or ground
- Module failed
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Scan all modules and review event data for other communication codes that could point to a shared network or power problem.
- Use HVAC live data and mode commands to check whether the rear mode actuator reports status or changes position when vent and floor modes are selected.
- Inspect the rear mode actuator connector and harness for looseness, corrosion, moisture, rubbing, or crushed wiring.
- Check the actuator's specified power and ground, including under load, before suspecting a network fault.
- Test the network wiring to the rear mode actuator for opens, shorts, or abnormal resistance using the vehicle diagram and manufacturer procedure.
- If the supply and bus circuit pass, follow the manufacturer's actuator communication test to determine whether the rear mode actuator itself has failed.
Common fixes
- Reconnect and secure the rear mode actuator connector.
- Repair damaged or corroded wiring at the rear actuator harness.
- Restore a faulty power or ground feed to the actuator.
- Repair an open or short in the network wiring serving the rear mode actuator.
- Replace the rear mode actuator if its power, ground, and network circuits test correctly.
Can I keep driving?
The vehicle is generally drivable, but rear air may not switch between the selected outlets. Check that the front defrost can keep the windscreen clear; seek diagnosis sooner if other network-related warnings or system failures occur.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.