Man I hate this stupid *** truck. First time I've used the heat in about a year last night and when I stop at a light the heat would blow cold air instead of heat, once I start going again it works fine. Any ideas?
Does the vehicle overheat? If so, then it is most likely a bad water pump impeller. If not, is the coolant low? If the system is full then you have a couple of options. Vacuum leak or restricted heater core.