When I set my temp control dial to heat?

When I set my temp control dial to heat (it is a manual temp control) even just a little and then turn it back down to cool the air coming out stays hot, the only way I can get it to reset so cool air comes out is by shutting off the truck and let it sit for some time and then go from there, I have replaced the dish console controls, which has not made a differences, is there something beyond that I need to replace or check.
Thanks Tony