U0166 code meaning
Lost Communication With Auxiliary Heater Control Module
What U0166 means
U0166 reports that the auxiliary heater control module has stopped communicating. Such heaters, burning fuel or working electrically, warm the coolant or the cabin air before or during a drive and are common on diesels and cold-climate packages. GM lists the code in the engine controller of the 6.6 L LML Duramax (14OBDG10): it fails when no message from the heater module has come in for more than 2.5 s after the ignition has been on for at least 3 s at 9-16 V, and it is a special Type C fault that does not light the MIL. The Saab 9-5 U-code list and Ford's 2014 Boss 302S code list also include U0166. U0164 is the main HVAC module. In practice the extra heater stays off, so warm-up and cabin heat take longer in winter.
U0166 at a glance
- Check engine lightUsually not
- Emissions testNot an emissions code
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0166
- Auxiliary or parking heater does not start
- Slow engine and cabin warm-up in cold weather
- Code stored in the engine or climate controller
Common causes of U0166
- Most likelyHeater module fuse blown or ground corroded (often underbody mounting)
- PossibleCorroded heater connector from road splash
- PossibleBus line to the heater damaged
- Less likelyAuxiliary heater control unit faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Auxiliary heater moduleCAN bus wiring
What each part of U0166 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 66 | The exact fault in that area. |
What to do next
- Note U0166 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0166
What does U0166 mean?
U0166 means "Lost Communication With Auxiliary Heater Control Module". U0166 reports that the auxiliary heater control module has stopped communicating. It is a generic code, so it means the same on every make.
Is it safe to drive with U0166?
U0166 does not normally turn on the check engine light by itself. Watch for the symptoms above and have it checked soon.
Will U0166 fail an emissions test?
U0166 is not an emissions code, so on its own it usually does not fail an emissions test. Other stored codes or a lit check engine light still can.
How hard is U0166 to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will U0166 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.