A capacitor is a small, but crucial, electrical component in your air conditioner. It stores electrical energy and releases it in a burst to help start the compressor and fan motors. Without a functioning capacitor, these motors wouldn't be able to start, and your AC wouldn't be able to cool your home. Common signs of a failing capacitor include difficulty starting the AC, humming sounds from the unit, or the AC unit turning on and off frequently. If you suspect a bad capacitor, contact a qualified HVAC technician to have it replaced.