2005 GMC Truck AC blows hot air after changing alternator

I changed my alternator yesterday, after reconnecting the neg battery post and starting the truck, the climate control read 74deg with the air conditioner in auto mode, the air was cold for 20sec then started blowing hot air. I heard that this could be related to the computer needing to be reset, and I could do this by disconnecting the neg battery post and letting the computer discharge. Tried this a couple of times and still no results.

Looking for any suggestions or help.