Why is my car AC blowing warm air?

Warm air from the AC vents (when cold is selected) means the AC compressor isn't running or there's no refrigerant for it to compress. Pop the hood with the AC on max — if the AC clutch on the front of the compressor isn't engaging, it's either out of refrigerant (the system locks itself out to protect the compressor), a blown fuse, or a bad pressure switch. If the compressor IS running but air is warm, the system is critically low or the expansion valve is stuck.

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