Turn the dial to select another vent for the air, not close the individual vents. You'll have to do this with the engine running. You should hear the blend door and vent selection door closing when you turn the dial from front, to mix, to lower, to low/def mix, and then to def only. If you're not hearing things opening and closing, you should verify that you have vacuum from the engine to the controls in the dash. Start in the engine compartment.
Before performing this test, I'd check to see which vents are currently blowing air to be sure you don't get a false test result. Then start moving the selector around.
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Moderating everything now, let me know if I can help. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.