I have an 02 F150, about a day or so ago, I have been having heating problems, there is none, only cold air blowing in. The Rad is full, temp gauge indicated engine at proper operating temp, coolant looks clean, thermostat is new.

 

Check your blend door actuator - I just finished removing mine on an 03, it was fried -
Do a search for blend door actuators and follow the instructions for diagnosing, some good pictures too, to be sure you're looking at the right gizmo.

 

Hi Doug

It was the door actuator, it had seized up on me.

 

I had the same problem on my 97'. Finally figured out that it was the actual blend door itself. Dealer told me it was actually a common problem. The points that the door pivot on crack and it keeps the door from opening and closing. Costed me about $300 to get fixed.

 

Would this be the same issue I am having with my 97 F150? Heat turns to cold air and the temp gauge reads low if anything and the coolant is forced into the fill tank and spills. I changed the Thermostat 2 days ago and ran great for a day then same issue.