This code indicates that the main vehicle computer has lost communication with the auxiliary heater control module, which manages the supplemental heating system. It typically points to a power, ground, or data network issue preventing the module from responding on the vehicle's network.
Internal electrical failure of the module itself is the most common reason for a total loss of communication.
Corroded, chafed, or disconnected wiring and connectors at the heater module can interrupt power, ground, or data signals.
A loss of power or ground due to a failed fuse or relay will prevent the module from turning on and communicating.
An open or short circuit on the communication bus wiring specifically affecting the heater circuit can cause this code.
A failure of the main engine computer is extremely rare and almost never the root cause of this specific code.
It is a supplemental heating device, often a small fuel-burning heater or electric element, used to provide cabin heat quickly. This is especially common in diesel vehicles which produce less waste engine heat.
The vehicle is generally safe to drive, but you will likely experience a lack of cabin heat and poor defrosting capability in cold weather.
No, replacing the PCM is almost never the correct fix for a U0166 code. Drivers are often misdirected toward the main computer, but the fault almost always lies with the auxiliary heater module itself, its power supply, or its wiring.
Yes, if the vehicle's system voltage drops too low, the auxiliary heater module may shut down and fail to communicate, triggering this code temporarily.