Heat, and cold air in the cab. on 1999 Ford Expedition

In the Cab the blower works fine, when I put the Heat on there was no hot air, warm not Hot I replaced the Thermostat, and now I have hot air, but when I switch the control to the Air Conditioner. It is is blowing the Hot air. I checked in the engine, and the air conditioner lines are cold, and working. What is wrong? Is there a door that opens, and closes going from Heat to Air, Please let me know. Thanks Again for the Advice. Don P.

Asked by for the 1999 Ford Expedition
There Is A Heater/ A/c Door At The Heater Core Need To First Check Vaccum Lines Because A Leak Can Also Be the Cause But To Get to That You Have To Remove The Dash Get One Of The Repair Books You Will Need It To Do The Repair...Hope This Helps!!!
2 more answers
Did you ever find out what was causing this problem? Was it the blend air door?
it going to be the blend door sticking...happened to me in my 2001 expy xlt. during -35c. 3hr drive home bundled up in my snomobile gear just to stay warm. at the time i had warranty and ford fixed it