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U0166 Lost Communication With Auxiliary Heater Control Module

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0166 reports that the auxiliary heater control module has stopped communicating. Such heaters, burning fuel or working electrically, warm the coolant or the cabin air before or during a drive and are common on diesels and cold-climate packages. GM lists the code in the engine controller of the 6.6 L LML Duramax (14OBDG10): it fails when no message from the heater module has come in for more than 2.5 s after the ignition has been on for at least 3 s at 9-16 V, and it is a special Type C fault that does not light the MIL. The Saab 9-5 U-code list and Ford's 2014 Boss 302S code list also include U0166. U0164 is the main HVAC module. In practice the extra heater stays off, so warm-up and cabin heat take longer in winter.

Likely causes

Heater module fuse blown or ground corroded (often underbody mounting)
Corroded heater connector from road splash
Bus line to the heater damaged
Auxiliary heater control unit faulty

Symptoms

  • Auxiliary or parking heater does not start
  • Slow engine and cabin warm-up in cold weather
  • Code stored in the engine or climate controller

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€40–1,200
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

auxiliary heater modulecan bus wiring

Related codes

U0164U0165U0325

References: gsi.ext.gm.com, saabwisonline.com, performanceparts.ford.com.