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If there was a blockage, you wouldn't get air through the vents on the mix selection. The problem doesn't sound like a blockage, but rather an issue with what is called a mode door. The mode door is what determines where the air flow goes. Now, on your vehicle, the mode door is actuated with a cable which is connected to a cam on the mode door and at the opposite end it attaches to the controller you adjust. I attached a picture of the cable and where it attaches under the dash.
What I would suggest is to inspect the action of the door when the cable is moved. Does the cable fully turn the door or is the cable loose or moving, causing the door to only partially move?
Take a look at the picture 1 below. Next, locate that portion of the heater box under the dash to see if it is working. On picture 2, I highlighted the cable I am referring to.
Check that and let me know what you find.
Images (Click to enlarge)
Saturday, September 21st, 2019 AT 10:36 PM