Randomly and without any discernable pattern my Buick Lesabre's (Custom trim) A/C will start to come out of the defrost vents. I'll have it set on either Bi-Level, or Max, or anything, and all of a sudden and for no reason it'll switch to defrost. The indicator light will say its where I wanted it, but it will come out of the front defroster vents. I can play with the buttons and sometimes it'll come back when I move a lucky dial, other times it just stays like that until it randomly clicks back to what its suppossed to be.

Google is inconclusive. Help. Please. This is Florida and I'm fucking broiling.

 

The HVAC doors are vacuum actuated. There is a vacuum line that runs off of the intake back through the firewall and up under the dash. When that vacuum line breaks the HVAC defaults to defrost. There should be a vacuum diagram under the dash that will show you which line controls the HVAC. Usually that line is either working or broke and not intermittent. It's something worth checking though.

 

Alright, first thing I'll look at later today when it cools down. Thanks for the direction. I suppose its possible its not broken, but is getting a shitty seal for some reason?