2001 Chevy Silverado Hot Air

Sometimes when I turn my temp. Selector to hot and then return it to the cooler sitting I still have hot air blowing from the vents. Now if the truck sets over night everything works fine. But while the truck is running or throughout the day I can turn the air cond. On and it will still blow out hot air. But like I said, after it sits over night it usually works great and the air is cold. I have been told it's a sensor, which is located behind the glove box? If it's a sensor where is it and what is it called and can I replace it?

Thanks