I had a similar problem, except mine was with the recirculate button. I had mice get into my heater/AC duct, and build a nest. When I went to change from recirculating air from inside the cabin to pulling in fresh air, an acorn or something got wedged against a flap in there somewhere. Might be your problem, or may be something as simple as a loose wire or blown fuse, or even a mechanical linkage disconnected or broken.

- Wolf