Code Definition

U0166 (87) — Lost Communication With Auxiliary Heater Control Module

System: Network Communication  |  Type: Generic (OBD-II Standard)  |  Make: Ford

About U0166

U0166 is a generic (obd-ii standard) network communication diagnostic trouble code recorded for Ford vehicles. It belongs to the U-prefix family of codes, which covers inter-module network communication (CAN, LIN, FlexRay, or similar) faults.

Scanning & clearing: Network (U) codes report communication faults between modules. When diagnosing U-codes, checking the vehicle's CAN bus integrity, supply/ground at each module, and reading all modules for related faults is usually the first step.

Because U0166 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The Body Control Module (BCM) and/or Heating, Ventilation and Air Conditioning (HVAC) control module communicates with the Auxiliary Heater Control Module over the Controller Area Network (CAN) to manage supplemental heating functions. DTC U0166 is set when required CAN messages from the auxiliary heater control module are not received within a calibrated time while the ignition is in the RUN position. Subcode 87 indicates a missing or invalid message condition, confirming a loss of communication rather than a data plausibility fault.

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