Contactors are the switch to turn on your heat pump or air conditioning compressor and condenser fan. The switch is an electrical switch controlled by your thermostat and air handler or furnace. The thermostat sends a low voltage signal to an electromagnetic coil in the bottom of the contactor. When energized the coil pulls a metal rod through the coil. On one end of this rod is a connector plate that closes the gap on the high voltage power lines running to the compressor and condenser fan closing the circuit and energizing the compressor and fan. Sometimes the low voltage coil in the contactor breaks or shorts out and does not create the magnetic field to pull the metal rod through the coil. Other times the plate the rod pulls becomes corroded or the contact points become pitted which can prevent a full contact from occurring and preventing the compressor or fan from starting.