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.
Medium severity. Can I keep driving?It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.
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
Most likely causes
- Bus wiring open or shorted
- Module without power or ground
- Module failed
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
What each character of U0664 means
- U: Communication network
- 0: Generic code: same on every make
- 6: Subsystem depends on the code range
- 64: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0664?
It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.
What causes code U0664?
The most common causes are: bus wiring open or shorted; module without power or ground; module failed.
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.