Compressor

The compressor is the pump inside the outdoor unit that circulates refrigerant around the system; without it, nothing cools. It's also the most expensive component to replace, so a technician who blames “the compressor” should first rule out cheaper culprits like a failed capacitor, low refrigerant or a tripped overload protector. A compressor that hums, trips the breaker or short-cycles on and off is worth a careful second opinion before you pay for a new one.