Rear Blower Turns On By Itself/Defaults to High Blower Speed and/or Defaults to Full Cold - keywords 04-01-39-009A cold control DTC head hi hot HVAC inoperative operation rear unwanted

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Owners may comment intermittently the rear HVAC blower may turn on by itself/default to high blower and/or the rear temperature blows full cold. When checking DTCs, none will be found. This concern could be caused by an open/high resistance/short to ground/etc. on circuit 2821 between the Front Auxiliary Control Head and the Auxiliary HVAC Control Module (SI Document ID # 1242204).

Circuit 2821 is a 5 volt reference being supplied to the Front Auxiliary Control Head from the Auxiliary HVAC Control Module. When this reference voltage goes low, due to open/high resistance/short to ground/etc, the feedback circuits (760, 2930, 454) change voltage and the control head becomes uncontrollable and will default to high blower and/or full cold.

Note: The rear blower may turn on by itself even if the rear HVAC controls are in the off position, but when the ignition key is turned off the blower will stop.

To correct this concern, inspect circuit 2821 for any opens/high resistance/shorts to ground/etc. The most common area of concern is a poor terminal tension at connector C1 terminal A1 (circuit 2821) of the Auxiliary HVAC Control Module.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.