A car AC compressor is responsible for circulating refrigerant throughout the air conditioning system. If it goes bad, the entire system will stop working. The most common symptom of a failing AC compressor is warm air coming from the vents when the AC is turned on. Other symptoms can include strange noises coming from under the hood, and leaks around the front of the engine. If your AC compressor does go bad, it will need to be replaced by a professional.