a/c goes from panel vents to defrost

[RB1953]

I have a '09 Xlt that will change from the front panel vents to the defrost vents when the a/c is on. It is manual controls. This will happen at random, from the panel to defrost and then back to the panel. Any ideas. I don't want dealer to quote some way out of sight price for something that might be simple. I am hoping the warranty will cover @ only 26000 mi and being a certified vehicle. Just trying to be prepared. Thanks

 

[mace_b]

check vacuum leak post. Pretty sure it is your vacuum check valve.