Lost Communication With Auxiliary Heater Control Module
The U0166 code means the vehicle's onboard diagnostic system has detected: Lost Communication With Auxiliary Heater Control Module. This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- $50 - $600 (fuses, grounds, or wiring most often; module replacement only after the circuit tests good)
Network codes mean modules are not communicating. You need a scanner that reads all systems to find the problem.
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Common Symptoms
- Check Engine or related warning light on
- The Auxiliary Heater Control Module's functions may be degraded or offline
- Often stored alongside other U-codes when a whole bus segment drops
- Symptoms can be intermittent - connector and wiring faults come and go with heat and vibration
Probable Causes (Ranked by Likelihood)
- The Auxiliary Heater Control Module has lost power or ground (check its fuses first) High
- CAN bus wiring damage - chafed harness, corroded connector, water intrusion High
- Failed Auxiliary Heater Control Module no longer transmitting Moderate
- Another faulted module loading down the bus Low
Frequently Asked Questions
What does U0166 mean?
U0166 means other modules stopped receiving messages from the Auxiliary Heater Control Module on the vehicle network. It is a communication fault - the module may be fine but unreachable.
Can I keep driving with U0166?
Usually yes for a short period. The affected function may be degraded, and intermittent wiring faults tend to worsen - diagnose soon.
Should I replace the module for U0166?
Not first. Most communication codes are power, ground, fuse, or wiring problems. Replace the module only after those test good - modules are expensive and often require programming.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
-
Scan ALL modules - the pattern of which modules report lost communication localizes the break.
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Check the fuses and grounds feeding the Auxiliary Heater Control Module before suspecting the module itself.
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If many modules are unreachable, test the CAN bus: about 60 ohms across CAN High/Low (two 120-ohm terminators) with the battery disconnected.
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Wiggle-test connectors along the affected harness while watching for the module to drop on and off.
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Only after power, ground, and bus test good should the Auxiliary Heater Control Module be suspected.
Sources 2
This page is built from documented references. Verify against your own service info before repair work.
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
- Cross-checked against the SAE J2012:2002 DTC table (2026-08-27)archived scan of the SAE standard — readable copy, not SAE’s current edition
Commonly Affected Vehicles
- All OBD-II vehicles - network (U) codes are standardized across makes
Related Codes
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