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.Google+
Car Problem Reports
GMC Yukon XL 2500 Heating and AC Temperature and Air Delivery Mode Door Actuators May Fail
GMC Yukon XL 2500 Problem
Visitor, 2004 GMC Yukon XL 2500, 85,000 mi
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!