The compressor is the heart of the refrigeration circuit. It pumps and pressurizes the refrigerant to move it through the A/C system. Compressors work hard and run hot, up to several hundred degrees and several hundred pounds per square inch of internal pressure, yet they rely on only a few ounces of lubricant to keep their parts moving. But if the lubricant is lost because of a leak, or the lubricant breaks down due to contamination, the compressor won't last. Sooner or later, the compressor will call it quits.

The most common symptom of a compressor failure (besides no cooling) is a seized compressor. It won't turn when the magnetic clutch engages, and you may hear squeals of protest from the drive belt. Or, the belt may have already broken or been thrown off its pulleys.

Loss of lubrication is unquestionably the most common cause of compressor failure. This can happen when there's a refrigerant leak somewhere in the system that allows refrigerant and oil to escape. Typical leak points are hoses, hose and pipe connections (O-rings and flange gaskets), the evaporator, condenser or the compressor shaft seal. An electronic leak detector or dye should be used to find the leak so it can be repaired.

A restriction inside the A/C system also can starve the compressor for oil. Oil circulates with the refrigerant, so if the orifice tube or expansion valve is blocked it may cause the compressor to run dry and seize.