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This can be caused by a few things. The most common being an actuator fault.
The best procedure would be to have the rear hvac controller scanned for fault codes. There is likely a fault code for the rear temp and/or mode actuator. Often the actuators themselves fail and need to be replaced. The other possibility is an over travel of the temp door. The door can flex over time causing the actuator to over rotate. A new software calibration was developed to compensate for this. I'd recommend scanning it for fault codes (again for the rear hvac controller) and if an actuator fault code... replacing the actuator and reflashing the controller. If you have further questions please ask away. I'll be back in a bit as I'm leaving work at the moment.
The actuators are not difficult and a relearn procedure is forced after simply a battery disconnect and voltage drain by touching the two cables together. I would be happy to send instructions. The right rear 1/4 trim needs to be partially removed and then it's basically 3 screws and one connector.
The controller reflash can only be performed by a repair facility with a GM Tis2web programming license.
Please note that after removing the sill plate and 1/4 molding and then the interior 1/4 panel screw(that often doubles as a cargo net attachment) the trim will pull out several inches and you can work through the opening from the rear of the vehicle. The entire panel does not need to come out. Just repositioned towards the center of the vehicle.
Auxiliary Temperature Valve Actuator Replacement Removal Procedure
Notice: In order to avoid actuator damage, DO NOT apply power to the actuator when it is not installed in the HVAC module.
Install the auxiliary air temperature actuator (1) to the auxiliary HVAC module.
Notice: Refer to Fastener Notice in the Preface section.
Tighten Tighten the screws to 1.6 N·m (14 lb in).