My defrost/air knob only stays in one position. When trying to turn it, the knob bounces back to to the defrost position.
There is a possibility that the mode door for the vent, floor, defrost blend door is broken in the evaporator case, or that something fell into the case through the defroster duct and is binding the door. The dash panel and center controls need to be removed to access the lever. I recommend you seek a certified mechanic, such as one from YourMechanic, to come and test your vehicle for you to see if they can see if something is binding the lever or cable and that the problem is not inside the evaporator case.
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