Why is my car AC not blowing cold air?

When the AC blows outside-temperature air with cold selected, the compressor either isn't turning or has nothing to compress. Open the hood with the engine on and max AC selected — if the small clutch on the front pulley of the compressor isn't clicking in, it's almost always the system locking itself out due to low refrigerant (a safety cutoff to protect the compressor). Second most common: a blown 10A AC clutch fuse. Third: a failed pressure switch on the receiver-drier. A DIY manifold gauge or a $40 refrigerant kit with a built-in gauge will tell you in five minutes which one it is.

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