Ford Expedition 2003
Might be related to your IAC valve: it takes care of your idle speed. If it isn't working well, your idle will be way low, possibly too low to run an AC. The IAC usually just needs cleaning which is an easy DYI job and is an inexpensive thing to try. It sits right on top of the engine and comes off easily by taking off a couple of bolts. You shake them a bit to get the gunk out and then spray into it with some air intake cleaner or carb cleaner before bolting it back on.
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