What would cause my heater to only blow out cold air in my 1999 Ford Ranger truck? I had a new therrmostat put in, and I had the heater core flushed out. My antifreeze is full. The truck does not run hot. It stays normal. It's strange because a few times I have had the heater off and would turn it on to try it, and it would blow hot air for about a minute, and then it would start blowing cold again. It doesn't do that every time but just once in a while. What is going on with it? Service technicians seemed to be stumped.