Air will blow from all vents (floor, defrost) except the center vents i dont know why. Help. I checked the vacume at the control switch and it seems fine

 

I'm having the exact same problem with mine now, how do you fix it?

 

I had the same problem on my 95 Blazer, the PCV valve was bad. My mechanic claims the vents get the vacuum source from the PCV valve and if it goes bad anything else vacuum operated will not work correctly.

 

If the vacuum to the control switch in the dash is "fine" (i.e. measures the same or close to the same as idle vacuum on the motor, then the vacuum system is operating properly.

To the OP, please provide your vehicle information.

 

I have the same issue. Not sure when it started. I can position the switch to vent or AC, and the air only flows out of the defrost vents and floor vents. Other than the PCV Valve, what else could be the issue?