One or more heater & AC (HVAC) air delivery and/or temperature mode door actuator may fail. This can result in incorrect air delivery or temperature. If this occurs, fault code(s) stored in the HVAC control module should be available to assist with diagnoses.Google+
Car Problem Reports
Chevrolet 1500 Pickup Heating and AC Temperature and Air Delivery Mode Door Actuators May Fail
Chevrolet 1500 Pickup Problem
Average Mileage: 110,668 mi (13,200 mi - 164,000 mi)
heating door is not opening.Where is the door located
Visitor, 1990 Chevrolet 1500 Pickup, 164,000 mi
heater will not blow hot air and everything is working but hot air is non existant
Visitor, 1991 Chevrolet 1500 Pickup, 125,474 mi
Took off old actuator and it did not any power so i filped the door around and the heat worked, so I tryed to find a actuator at parts house but never got the right one, i am still looking for the right one .
Visitor, 1995 Chevrolet 1500 Pickup, 140,000 mi
When moving selector knob from its current position to any other position, mode door makes a clicking sound as it attempts to move. Haven't replaced it yet as I still need to check if the door is stuck. Even after freeing a stuck, still a good idea to replace motor.