What it means
A control module is not receiving expected messages from the Auxiliary Heater Control Module. Auxiliary heater functions may be unavailable; the code alone does not identify why communication was lost.
How it works
The auxiliary heater control module manages an additional heating system and exchanges status or control messages over the vehicle network. U0166 sets when a receiving module does not get the expected messages from that auxiliary heater module within the required time; the fault may be its power supply, CAN connection, or gateway routing.
Common causes
Most likely first.
- Auxiliary heater unpowered
- CAN wiring to auxiliary heater broken
- Gateway module routing CAN messages incorrectly
Symptoms
- Auxiliary heater functions may be unavailable
- Other communication codes
How to diagnose it
- Scan all modules and check freeze-frame or event data for other communication faults or a low-voltage event.
- Check whether auxiliary-heater functions are available and whether the auxiliary heater module responds to a full-module scan.
- Inspect the auxiliary heater module's power, ground, CAN wiring, and connectors for looseness, corrosion, damage, or water entry.
- Test module power and ground under load against the manufacturer's specification.
- Test CAN communication at the auxiliary heater module and check gateway routing using the vehicle's wiring information.
Common fixes
- Secure or clean a loose or corroded auxiliary-heater connector.
- Repair damaged CAN, power, or ground wiring to the auxiliary heater module.
- Restore the module's failed power or ground supply.
- Correct gateway routing or configuration if network tests identify it.
- Replace the auxiliary heater module only after its supplies and network connection pass testing.
Can I keep driving?
If the fault only disables the auxiliary heater, basic driving is generally unaffected, but extra cabin heating may be unavailable. Do not assume the auxiliary heater can clear a fogged windscreen; use working defrosting controls and address any broader electrical symptoms.
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.