..............need help..........

2002 1500 heat has been fine now i can start the truck to let it warm up and still no heat until i press the gas...what the h_ll is going on..i can drive this truck 40 miles and have good heat stop at a stop light or somewhere to run in a store or something and let it idle and no heat..any idea ???? sorry for the hi jack oldkimmer thanks mike:wack: