Q: My A/C get "cold" even at lowest setting, and back vents blow even hotter air. on 2003 Ford Expedition

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It has been hot since late spring, and we have had triple digit heat in my city for weeks now, but my trusty 2003 Expedition has previously handled this kind of weather like a champ! The Ford dealer serviced my car early last month(105K service!!) and I was told my A/C didn't need a charge even though I told them that I didn't feel it was cooling as effectively as before. The service department said "it can't cool as well when the outside temps are this high", and I just accepted that. But for the past two days, the air it blows is barely cool! I was told by a Sears Service tech today that I need a new "head control" installed, and failure of that part is fairly common. He said I should take it back to Ford! What does this mean, and what should I expect it to cost? I can't take this heat without my Expedition functioning up to par!! I would appreciate any insights.
(1) Answer
If he means your a/c head controller it is the part that you turn your a/c on with in the dash. You can simply install this yourself if you were to buy the part.