After receiving the vehicle, the fault code is read by the fault detector first, which is 21 (oxygen sensor fault). After the replacement of the oxygen sensor, the trouble code is cleared, but the failure is still the same. A further examination of the engine revealed that the exhaust pipe was rhythmically "burst", and that there would be a shooting phenomenon when the engine speeded up. The vacuum meter is used to detect the intake manifold vacuum degree at idle speed. The vacuum table pointer is indicated at 45 kPa-68 kPa, with irregular rise and fall, and the swing amplitude is also larger. According to the above analysis, it is considered that some cylinders may not work well or do not work. One by one cylinder breaking test, when the 3 and 4 cylinders were broken, the working state of the engine did not change, which proved that the 3 and 4 cylinders were not working well. The two cylinder spark plugs were checked and no bad points were found. Then the 1, 6 cylinder ignition coils and the 3 and 4 cylinder ignition coils were tested, and the 1 and 6 cylinders were still working well, indicating that the fault was on the first side line of the 3 and 4 cylinder ignition coil. The resistance of the multimeter is used to detect the state of the first side of the ignition coil of the 3 and 4 cylinders, and the state of the circuit is short circuited. Inspection showed that a section of the line had been affected by high temperature and aged, and the two wire cores had been contacted many times. After the replacement of this line, the fault is excluded. After cleaning the oxygen sensor, the oxygen sensor is reinstalled to the car, and the engine work is still good and no fault code appears. It shows that the oxygen sensor itself has no fault, but because the mixed gas that is not ignited in the 3 and 4 cylinders is ignited by the high temperature gas after the discharge of the cylinder, so that the gas in the exhaust pipe is not correct. It often burns, resulting in transient faults and exhaust pipes of oxygen sensors. In addition, the 3 and 4 cylinders work badly, reducing the number of effective working cylinders of the engine and reducing the vacuum degree of the intake manifold, which affects the normal operation of the engine at idle speed, acceleration and high speed.