Need help with climate control to avoid $200 diagnosis fee at dealer!

[willrev]

Well, warm weather is approaching in NC. The car is covered in yellow pollen so I don't want to ride with my windows down.

When I run the AC - I hear the climate control relay click and then the center vents quit blowing AC and it starts blowing for 20 seconds out of the defrost vents. Then it switches back. This goes on continually.

I think I posted this before, but cannot find the post. I have searched for all my posts and cannot find it.

I replaced the climate control unit last year. I think it is under warranty. My parts guy says it isn't the unit. He says it is a vacuum leak. But if so, why would the relay clicking right before it changes? I would think if a vacuum acctuator or one of the things on that crossover block had gone bad that it would either stay open or shut.

My question - could it be the temperature sensor over the rear view mirror? It switches between vents even if I have the temp wheel all the way to bypass the automatic feature.

I need to know where to start with this. The dealer wanted $200 just to diagnose it. That is rediculous!