The contactor is a high-voltage relay that acts as the switch to turn the outdoor compressor and fan motor on and off. When the thermostat sends a 24-volt signal, a magnetic coil inside the contactor pulls a set of electrical points together to complete the 240-volt circuit. Over time, these points can become "pitted" or "burnt" due to the electrical arc that occurs every time the system starts. A failing contactor may hum, chatter, or even weld shut, causing the unit to run continuously. It is one of the most frequently replaced electrical parts in an air conditioner and is a staple item in any technician's truck stock.