Why is my AC compressor not engaging?

Look at the front of the compressor with the engine on and AC set to max cold — the center of the pulley should click and start spinning with the belt. If it never engages, walk down the checklist in order: (1) low refrigerant — the low-pressure cutoff switch blocks the clutch to protect the compressor, this covers ~70% of cases; (2) blown AC clutch fuse (usually 10A, in the underhood fuse box); (3) failed pressure switch on the receiver-drier; (4) burned-out clutch coil or worn air-gap; (5) failed AC relay. A DIY manifold gauge or a $40 refrigerant kit with a built-in gauge diagnoses #1 in five minutes.

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