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Under normal conditions, all pipes and connections in the refrigeration system should be dry and free of oil stains. If there is an oil stain in a certain area, it indicates a minor or severe leak there. For example, a Fokang 988 sedan with the license plate number Yu H27828, manufactured at the Wuhan plant of Second Automobile Works in 2007, had traveled over 166,000 kilometers; it developed a problem where the air conditioning did not work properly during the summer of 2010. The driver took the car to a repair shop for repairs, and after checking, the mechanic found that there was a severe shortage of refrigerant. After releasing the remaining refrigerant, the technician conducted a pressure test for leaks in the air conditioning system, applying pressure of 1.2 MPa to both the high-pressure and low-pressure circuits for 30 minutes; the gauge needles did not move, and no oil contamination was found at any of the connections in the relevant circuits. After releasing the gas that had been added, the technician created a vacuum, then filled the system with 900 grams of refrigerant, and the car’s air conditioning system returned to normal operation. But 3 days later, the problem of the air conditioner not cooling up reappeared. After inspection, the technician found that the reason for the lack of cooling was again a severe shortage of refrigerant. The technician inspected all the pipe connections in the air conditioner and found no areas with oil contamination, so he came to me for help. Upon careful observation, the author found slight oil stains on the clutch disc at the front end of the compressor. Therefore, it is recommended that the repairman either replace the compressor or remove the compressor and replace the front air seal. Finally, after the repair technician replaced the compressor, the refrigerant deficiency issue was resolved. After removing the front electromagnetic clutch and pulley from the removed compressor, it was found that there was a noticeable layer of oil residue on the inner wall of the compressor’s front end. This compressor does not leak air when at rest, but during operation, due to a slight defect in the front air seal, the air seal will experience slight leakage under the action of the rotating shaft, thereby causing leaks in the sealed air conditioning system. If the leakage here is severe, the refrigerant will run out after it has been added, in not much time. It is because the leakage here is mild that the refrigerant leaks away slowly over the course of 3 days, as the compressor is operating. The reason why there is only slight oil contamination on the surface of the front clutch disc, while there is significant oil contamination on its inner wall, is that the surface of the front clutch has good ventilation, allowing most of the leaked oil vapor to be blown away by the circulating air. In contrast, the inner wall of the front part is covered by the pulley, so the oil vapor cannot be easily blown away by the wind; as a result, more oil accumulates there. Moreover, because of the pulley’s obstruction, this accumulation is not visible.