GMC Troubleshooting Questions? Ask a Mechanic for Answers
Does it change at all based on the mode setting? Defrost/vent/floor?
It sounds like the mode door actuator has gone out. It's job is to move a control door under the dash from one position to another based on where you want the air to blow out from. Do you want to replace it yourself? It's not very hard.
Here is a diagram that shows how to replace it. It is under the dash on the driver's side.
ACDelco 15-73596 or GM 89018675
Please let me know if you need additional info.
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