Related to the model: FAW Volkswagen 1.8 L car. Malfunction: the engine is unstable when the refrigerator is working, and the speed is 680 r/min-860 r/min jitter. Cause of failure: failure of oil and gas separator causes air leakage in intake system. Preventive measures: replacement of oil and gas separator. Summary and suggestion: fault diagnosis is done through fault code and data flow analysis. (1) the use of VAS5052 detection, there are 1 cylinder system too dilute, as well as a number of cylinder fire, idle speed of the air control system is higher than the standard value of the fault code. (2) according to the fire failure code, entering the 01-08-15 display group, it can detect the alternately rising of each cylinder, verifying the existence of the serious unstability of each cylinder, indicating the phenomenon of fire breaking, oil breaking or abnormal combustion. (3) further read to the 01-08-004 area, display as "00110111", first for memory fault memory, fifth for A adjustment, normal display of 0, indicating the adjustment failure, the third area is 0%, also indicated that the adjustment has been closed by the electronic control unit. (4) the intake volume may change from 1.9g/s to 2 g/s, smaller than the standard value (2.3 g/s to 2.5 g/s); the load is changed in 12%-15%, the injection pulse width is 1.02 ms-1.28 MS and the throttle opening is 0%; thirty-second data group 1 shows that the self learning value before the engine is 6% at the engine idle speed, indicating that the mixture is short term concentration adjustment. The above data flow analysis shows that there is a leakage phenomenon in the intake system.