* Faulty AC compressor clutch relay: This relay controls the power to the compressor. If it's stuck closed, the compressor will continue to run even with the ignition off.
* Stuck AC compressor clutch: The clutch itself might be mechanically stuck, preventing it from disengaging. This is less common but possible.
* Problem with the AC control unit: A malfunctioning control unit (either the buttons or the internal electronic module) might be sending the wrong signals, keeping the compressor engaged.
* After-run system (rare): Some luxury vehicles have a brief after-run system for the AC to cool down the cabin, particularly after a hot drive. This isn't usually a long run time, but a malfunction could extend it.
* Wiring issue: A short circuit or faulty wiring could be providing power to the compressor even after the ignition is off. This could be anywhere in the system's wiring harness.
* Low refrigerant: Ironically, a low refrigerant level *can* sometimes cause the compressor to run continuously, as it tries to compensate. However, this is usually accompanied by poor cooling performance.
Important Safety Note: If your AC compressor continues to run after the car is off, it's crucial to address this immediately. The continuous operation will drain your battery and potentially damage the compressor itself. It's best to disconnect the battery's negative terminal to prevent further issues until you can have the car inspected by a qualified mechanic.
