There is a bulletin for a/c concerns, Have your dealer check into this:
#PIT5115: Erratic HVAC Operation C67 Or C68 - (Aug 30, 2011)
Subject: Erratic HVAC Operation C67 or C68
Models: 2011 Chevrolet Equinox
2011 GMC Terrain
With No Codes or B101E 4B
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:
Customers may comment on an intermittent HVAC operation when changing modes or unwanted temperature changes or A/C operation such as A/C turns off unwontedly and can be turned back on. During diagnostics the technician may not be able to duplicate the concern due to the intermittent condition. Using GDS check the HVAC control module for any diagnostic trouble codes (DTCs). It has been found that this condition may not set any DTCs, however you may find a DTC B101E 4B for a calibration not set or may have set after performing a HVAC Actuators Learn. This concern may be caused by a poor connection within the actuator wire harness.
Recommendation/Instructions:
When performing diagnostics if the fault is current for one mode not working, Example: The temperature mode is inoperative or may not fully sweep, Perform the normal Service Information diagnostics this may lead just to a faulty actuator. If the condition is intermittent and after checking the connections at the HVAC module and the actuators the cause of the fault is not found. Replace the wire harness from the HVAC module to the actuators/evaporator sensor and apply dielectric grease as per Service Bulletin # 09-06-03-004D for repairing fretting corrosion.