U0166
Lost Communication with Auxiliary Heater Control Module
The Auxiliary Heater Control Module has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. The auxiliary or parking-heater control is lost. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.
- Check engine light: Not always
- Emissions related: No
- Repair difficulty: medium
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
Common causes
Ordered by how often each turns out to be the cause.
- Auxiliary heater unpoweredCommon
- CAN wiring to auxiliary heater brokenPossible
- Gateway module routing CAN messages incorrectlyLess likely
Parts involved
- Auxiliary Heater Module
- CAN Bus Wiring
U0166 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.