A/C air flow keeps switching from "vent" to "defrost"


My 2002 F-150 AC air flow keeps switching back and forth between the “vent” setting and the “defrost” setting. It blows cold air just fine but I can’t really control which vents it comes out of. Why is it doing this?


Did you notice if it switches from one to the other when you hit the accellerator or go up a hill? If it does, it’s almost certainly due to a vacuum leak.



Acceleration does not affect it. If I set the blower on “high” or “Max AC” it will only blow out of the defrost vents.


I suppose it could still be a vacuum leak. You could look under the hood for any obvious unattached or broken hoses, though I don’t know if that particular model has anything obvious out in the open.



Replace the check valve between the intake manifold and the HVAC system, as well as the hoses.



there should be vacuum check valve going into fire wall this holds vacuum for vacuum motors under dash under hard exceleration. $6.00 should fix this.