What U0166 means
Lost Communication With Auxiliary Heater Control Module
How to diagnose
- Read freeze frame data. When U0166 set, your ECU snapshotted every sensor at that moment. This tells you what the car was doing when the fault triggered — RPM, load, coolant temp, throttle position, fuel trims. Almost always enough info to figure out the cause without further testing.
- Check for related codes. If you have U0166 plus other codes, fix the earliest one in the chain first — a single root cause often triggers multiple codes.
- Cheap fixes first. For most codes, the order is: inspect connectors + wiring (free), check for vacuum leaks (cheap), replace sensor (moderate), replace mechanical part (expensive). Follow that ladder before dropping money on parts.
Manufacturer-specific descriptions
Different manufacturers use U0166 for slightly different failure modes:
| BMW | Lost Communication With Auxiliary Heater Control Module |
Read U0166 on your own car
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