A Changan Mazda 3 1.6L AT car with a mileage of about 6000km. Car owner's reflection: when the vehicle is running, there is white smoke coming out of the engine room cover. Failure analysis: visual inspection is conducted after receiving the vehicle to confirm that there is no refrigerant leakage. It is found that there are many oil stains on the guard plate of the engine, and the air conditioning pipeline is checked. The pressure of the air conditioning system is measured and the pressure in the system is very low. It is judged that the refrigerant is leaked from the relief valve on the compressor. The pressure relief valve will only open when the system pressure is too high. What causes the air conditioning system to have too high pressure? There are two reasons for this analysis: the cooling fan does not work or the compressor does not shut down. First, connect the IDS fault diagnosis instrument to detect, and find that no fault code exists. Then refrigerant was added to the system to observe the high and low pressure line pressure, fan and IDS data flow were normal when the A/C relay was turned on. The A/C relay is closed to keep the engine idling, and the high pressure continues to rise to 0.35 MPa. Observation at this time found that the compressor was still in the state of suction. At this time the problem has been obvious, as the air conditioner is open when the compressor is running, but the fan is not working, causing the pressure of the system to rise, after reaching the safety pressure value, the pressure relief valve is opened, the pressure of the pipeline is released and the refrigerant is emptied. Next, check the compressor control circuit and use multimeter to measure the A/C relay. It is found that the relay lines controlled by PCM often connect with the relay. Check the line, there is no short circuiting iron phenomenon. Thus the fault of the electronic components in the PCM is judged. After dismantling the PCM of the defective vehicle, one of the transistors was destroyed. Troubleshooting: replace PCM and match it with special instrument, troubleshooting. Maintenance summary: through the maintenance of the above fault, the author thinks that the fault always has its law and cause, and the maintenance personnel can find out the fault only by careful examination and careful observation and testing. For similar failures, maintenance personnel need to check and test patiently and carefully, and eventually they can find problems.