What it means
The vehicle bus reports loss of communication with smart rear HVAC air cutoff actuator (shuts the rear duct entirely off). Cutoff cannot be commanded; rear airflow stays in last-known position.
How it works
The rear HVAC air cutoff actuator is a smart component that closes or opens the rear duct to stop or allow airflow. U0664 sets when the vehicle network no longer receives its messages, so the cutoff cannot be commanded and rear airflow may stay in its last-known position.
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 for related network codes and review event data for a shared communication or low-voltage event.
- Check HVAC live data and command the rear air cutoff on and off to see whether the actuator reports status or responds.
- Inspect the cutoff actuator connector and harness for a loose fit, corrosion, moisture, chafing, or pinched wires.
- Verify the actuator's specified power and ground under load to rule out a supply interruption.
- Test the network wires serving the cutoff actuator for opens, shorts, or abnormal resistance using the wiring diagram and manufacturer procedure.
- If power, ground, and network wiring are correct, use the manufacturer's communication test to confirm whether the rear cutoff actuator has failed.
Common fixes
- Reconnect and secure the rear air cutoff actuator connector.
- Repair damaged, corroded, or chafed wiring at the cutoff actuator.
- Restore the actuator's failed power or ground connection.
- Repair an open or short in its network wiring.
- Replace the rear air cutoff actuator after its supply and bus circuits pass testing.
Can I keep driving?
Driving is generally possible, but rear airflow may remain on or off regardless of the control setting. If rear cabin comfort or ventilation is affected, have the fault diagnosed; investigate promptly if other vehicle systems also lose communication.
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.