Why does my car AC make a clicking noise?

Where the click comes from tells you the fault. A click from under the hood every few seconds, with the cold coming and going, is the compressor clutch short-cycling — the system is low on refrigerant and the low-pressure switch keeps cutting it out. A rapid clicking or ticking from behind the dash, especially when you change temperature or vent settings, is a blend door or mode door actuator stripping its plastic gears. A single click on startup and then nothing is normal clutch engagement.

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