Air Conditioning problem
1994 Ford Explorer 6 cyl Four Wheel Drive Automatic 164000 miles
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Sometimes the heat/air comes on and sometimes it does not. I took it to the ford dealer it came on when they started the truck and told me nothing was wrong.

Intermittent problems are difficult to trace.
What exactly isn't working the fan or the compressor or other?
Is this happening for both heating and cooling?
Begin by looking for loose connections. In my Explorers, I have had to change several relays. You might have a bad blower relay or resister assembly.