The compressor is the heart of the A/C system, even though its only job is to pump oil and coolant through the system. It sucks refrigerant and oil in on the low side (or suction side), compresses it, and sends it out the high side (or discharge side). Compressors typically die due to a leak in the system. The oil and refrigerant constantly pump through the compressor, and a leak will release both. A compressor should last 150,000 miles, as long as it is well-lubricated. You can check the backlash a compressor has by grabbing the nut on the input shaft and turning it back and forth. Whether you are checking your existing compressor or one you are pulling out of a junkyard, if it has much more play than a new one at a parts store, you might be better off with a newer one.