One or more heater & AC (HVAC) temperature and/or air delivery mode door actuator may fail, resulting in improper temperature or air delivery. If this occurs, fault code(s) should be stored in the HVAC control module which should assist in diagnoses.
Improper HVAC Temperature and/or Air Delivery Due to Failed Mode Door Actuator on GMC Yukon XL 2500
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HVAC air flow restricted to defrost vents on top of dash, but AC flows to rear of vehicle via roof vents. However, when using heater, heat is only obtained through dash defrost vents and not the dash front, floor, or rear of the vehicle.
A/C will not blow cold while at idle or low speed driving. highway driving is just okay.
Driver's side A/C switches from cooling to full-on heat, but thermostat seems to work, because it increases fan speed to attempt to lower cabin temperature, by blowing blazing hot air!