U0166

Universal

Lost Communication With Auxiliary Heater Control Module

Definition source: General OBD-II reference catalog
Definition verification: Not independently verified for display.
Time: Varies by diagnosis and vehicle | Diagnostic difficulty: Medium
Medium Severity

The vehicle has detected that it cannot communicate with the auxiliary heater control module over its communication network. Depending on the vehicle, this module manages supplemental heating to help warm the cabin. The fault may reduce or disable auxiliary heat, but it does not by itself prove the module has failed; a power, ground, wiring, connector, or network problem can also interrupt communication.

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⚠️ Before You Begin (Safety First)

This fault is generally not an immediate reason to stop driving. If the loss of auxiliary heat leaves the windshield fogged or iced so visibility is poor, do not drive until visibility is clear. Follow any separate vehicle warnings, and have unusual electrical smells, smoke, or signs of overheating checked promptly.

Common Symptoms

  • The auxiliary heater may not operate or may provide less supplemental cabin heat.
  • The cabin may take longer to warm in conditions when auxiliary heat would normally assist.
  • A warning message or diagnostic trouble code may appear, depending on the vehicle.
  • There may be no noticeable symptoms if the auxiliary heater is not needed or another heating function remains available.

Common Causes

  • A loose, damaged, corroded, or poorly connected module connector may interrupt communication.
  • Damaged wiring or a fault in the vehicle communication network may prevent the module from responding.
  • Loss of module power or ground can make the module unavailable to other vehicle systems.
  • Low or unstable vehicle electrical supply may affect module communication.
  • The auxiliary heater control module itself may have an internal fault, but this should be confirmed after circuit checks.

Diagnostic Steps

  1. Check the dashboard for related warnings and note whether cabin heating or defrost performance has changed; this helps establish the practical effect of the fault.
  2. With the vehicle parked safely and switched off, look for obvious damage or looseness at accessible auxiliary-heater-related connectors and wiring. Do not disconnect components unless the vehicle-specific service information says it is safe.
  3. Check the vehicle battery and charging system for obvious problems or related warnings, since electrical supply issues can affect module communication.
  4. If a suitable scan tool is available, record other codes before clearing anything and check whether the auxiliary heater module is reachable. A scan result can help distinguish a communication interruption from a heater performance issue.
  5. If the code returns, the module is inaccessible, or wiring checks require circuit testing, have a technician follow the vehicle-specific wiring diagram and diagnostic procedure.

When to Call a Professional

Use a scan tool capable of checking the relevant vehicle modules to confirm which modules are communicating and whether related communication or power faults are present. Check the auxiliary heater module's power, ground, connectors, wiring, and network circuit using the vehicle-specific service information before condemning the module. Exact procedures vary by model, year, and engine.

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Commonly Asked Questions

Does U0166 prove that the auxiliary heater control module is defective?

No. It indicates lost communication with the module. Its power supply, ground, connectors, wiring, and the communication network should be checked before deciding whether the module needs replacement.

Can I drive with this code?

Usually, the code alone does not require stopping. Do not drive if reduced heating or defrosting leaves the windshield unclear, or if another warning indicates an immediate safety problem.

Will this code always make the cabin heater stop working?

No. The code concerns communication with the auxiliary heater control module. The effect on cabin heating depends on the vehicle's heating design and whether other heating functions remain available.

What helps distinguish a network fault from a failed module?

A technician can check for related codes, confirm whether other modules communicate, and test the auxiliary heater module's power, ground, connectors, wiring, and network circuit using vehicle-specific service information.

Browse Available Code Guides

Previous and next available guides in database record order, across makes. Adjacent entries may share a code number and do not necessarily describe related faults.