U0166
Lost Communication With Auxiliary Heater Control Module
Communication network
Generic OBD-II
Low
What it means
U0166 means the vehicle's network has stopped receiving expected messages from the auxiliary heater control module. That module runs the supplemental cabin heater (common on diesels and cold-climate vehicles) that adds heat when the engine alone isn't enough. You lose the extra boost heat in cold weather; the vehicle otherwise drives and heats normally.
Low
Can I keep driving? Yes, it's safe to drive — you'll mainly notice slower or weaker cabin heat in cold weather. Primary heating and driving are unaffected.
Common symptoms
What you'll notice
- Warning light or message related to the affected system
- The auxiliary heater control module's features stop working or work intermittently
- Stored 'lost communication' code in the module that reports the fault
- Weak or slow cabin heat in cold weather
- Auxiliary/supplemental heat not activating
Common causes
Most likely first
- Power or ground fault feeding the auxiliary heater control moduleLikely
- Damaged wiring or a loose/corroded connector at the auxiliary heater control moduleCommon
- Break in the CAN/bus wiring between the module and the rest of the networkCommon
- The auxiliary heater control module itself has failed internallyLess often
- Low battery voltage or a recent battery disconnect knocking modules off the busLess often
How to diagnose & fix
Work through it in order
- Check battery voltage and charge; a weak battery or recent disconnect is a common, easy cause.
- Scan all modules and confirm whether only the auxiliary heater control module is missing or several are — that narrows wiring vs. module.
- Verify power and ground at the auxiliary heater control module's connector.
- Inspect the connector and harness to the auxiliary heater control module for damage, corrosion, or backed-out pins.
- Check for a manufacturer software update or reflash; if wiring and power are good, replace the failed module. Clear and re-test.
FAQ
Quick answers
Is it safe to drive with U0166?+
Yes, it's safe to drive — you'll mainly notice slower or weaker cabin heat in cold weather. Primary heating and driving are unaffected.
Can I just clear U0166?+
You can clear it, but if the module is still offline the code returns as soon as the network looks for it. Fix the power, wiring, or module first.
Related codes
Often seen with U0166
Read & clear U0166 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX