Yesterday I evidently didn't get the key turned off all the way and ran the battery down on my Chevy duramax. Now when I put air on it will only come out the defroster slots no matter where I put the control knob.

I am clueless.

You got me. That sounds strange. I am sure this is a stupid question but did you check the fuses?

You might try disconnecting the battery for 15 to 20 minutes to "reset" the computer. I've heard of this but never experienced myself. I was told that disconnecting the battery would reset everything to defaults. It's worth a try.

Or just pull the HVAC fuse in the underhood fuse block for about 5 to 10 minutes to reset the system. When you reconnect the battery or install the fuse just start the vehicle and let it idle for a few minutes before touching any of the buttons or knobs to let it cycle through its positions. You may have to do it several times. GM actuators are a pain in the arse at defaulting to certain positions.