1998 Ford Explorer

Heater problem
1998 Ford Explorer V8 All Wheel Drive Automatic 125000 miles

The heater blower works properly, the air does not heat. It appears the blend door is not working. How do I confirm that is the correct diagnosis. What else could cause the failure to heat?
January 23, 2010.

Have you checked for a sticking thermostat, a clogged heater core or the heater control valve?

Jan 24, 2010.
Thank you! The engine builds heat, and maintains a constant temp, so im not thinking its the thermostat. But i did not consider a clogged heater core or a stuck valve, thank you! Im thinking its time for a trip to a pro to make sure. This truck has an electonic thermostat that is manipulated from the steering wheel. The air switched from defrost to heat to vent as dialed. The air cond also comes on as dialed, so the control valve or clogged heater makes sense. What does make it sound like the blender door is that it made a popping noise (according to the former owner) at random times which caused the air to turn warm.
ANY further thoughts would be extremely welcome, thanx!

Jan 24, 2010.