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Start by turning the ac off with engine running. Engage the ac and listen for the compressor to kick in. The should be a noticeable change in engine sound and a brief drop in rpm's. If nothing is heard, check the fuse. If fuse is good and you don't hear the compressor kick in, its most likely the clutch. Next step would be to disconnect the clutch wires and verify with a volt meter that the clutch is getting voltage. If no power at clutch wires, could be wiring.

Most likely the fuse or a bad clutch...or a bad ac guy. Low coolant would also stop the clutch from kicking in.

One other cause could be that the evaporator coil is dirty and clogged with debris, but I'm not exactly sure where its at. Should be somewhere behind the dash on passenger side near the heater/ac fan.