* Battery Drain: The AC compressor requires significant power. Without the alternator charging the battery, the battery will quickly drain, potentially leaving you stranded. Modern cars have better battery management, but this is still a significant risk.
* Damage to the AC System: Depending on the vehicle and the AC system, attempting to run the compressor without the engine may cause damage to the compressor itself or other parts of the system due to lack of sufficient power or incorrect pressures.
While some newer vehicles might have features like a battery-powered AC system that allows this for short periods, this is the exception rather than the rule. For most cars, it's best to always run the AC with the engine on.
