Busted blend door or the blend door motor. That is if the cooling system is good to go. When it has run for a while, touch the heater hoses and see if they are hot, both should be. If only one is hot the the heater core is stopped up inside. I do not envy any man that has to do a heater core in a Expedition. The first one I swapped out took 12hours. After 5-6 more I can do them in 3-4. This is using power tools too. My vote in the blend door motor is not working, either circuit wise or the motor is just bad.