U0166

What it means

The Auxiliary Heater Control Module has stopped transmitting on the vehicle data bus, so other control units no longer receive its messages. The auxiliary or parking-heater control is lost. Typical causes are loss of power or ground at the module, an open or shorted CAN/LIN bus wire, a connector fault, or an internal module failure.

Is it safe to drive?

The vehicle is usually drivable for short distances, but the problem can worsen or affect emissions. Diagnose and repair as soon as practical.

Most likely causes

• Auxiliary heater unpowered

  high

  One of the most commonly reported causes for this code — check this first.

• CAN wiring to auxiliary heater broken

  medium

  A plausible cause — test and verify before replacing parts.

• Gateway module routing CAN messages incorrectly

  low

  Less common, but worth ruling out after the more likely causes.

Related help

• Full U0166 diagnosis, causes & costs
• How to read check engine light codes
• Is it safe to drive with the light on?