The Chevrolet Cruz car with a mileage of about 10 thousand km in 2013. User response: the car was dragged to another 4S shop for maintenance shortly after driving accidentally. After 3 days of vehicle repair, the engine failure lights were on, so the user sent the car back to the store for inspection. The maintenance personnel use the special fault diagnosis instrument GDS2 detection, found the fault code P0171 - fuel correction system is too thin, but after the maintenance of the store, the failure is still not excluded. Inspection and analysis: I asked the user about the previous maintenance process. First, the maintenance staff of the shop measured the fuel pressure of 385 kPa, within the normal range of 345~414 kPa. Therefore, it is suspected that the gas leakage caused by the mixture is too thin, and the intake manifold, the carbon tank solenoid valve, the valve chamber cover and the connecting pipes are checked. There are no anomalies and no air leakage sound is heard. Taking into account the possible influx in the fuel tank after the vehicle fell, the fuel tank was checked and found to have a small amount of water and sediment, so the fuel tank, the oil road and the injector were cleaned and the fuel filter was replaced, but the fault lights went on again when the car was tested. It is doubted that the pressure regulating valve in the fuel pump may sometimes become stuck, so the fuel pump is replaced, but the trouble remains. Then, the maintenance personnel of the store have replaced the injector, air flow meter, air inlet pressure sensor, carbon tank electromagnetic valve, oxygen sensor, oil filling cover, valve chamber cover and vacuum auxiliary control valve and so on, all failed to eliminate the failure. Analysis of the above maintenance process can be found that the previous maintenance personnel did not have a clear fault point, and began to replace parts blindly to carry out the test. The author thinks that if we want to make clear the fault point, we should start with the data flow related to the fault code P0171. Therefore, the data flow is read by the connection diagnostic instrument. The long-term fuel correction value is almost up to the maximum. The idle speed is 23, and the acceleration pedal in the original ground changes to about 10 when stepping on the accelerator pedal. Compared with the same model normal car, the intake volume and fuel injection data (Table 1), the intake volume of the fault car is less than 2.23 g/s of the normal car, but the injection pulse width is very different. From this, we can restore the process of failure. First, the air flow meter of the fault car monitors that the intake volume is 1.73 g/s, so the oil is injected according to the intake volume, but the oxygen sensor detects the mixed gas too thin, so the engine control unit increases the long-term fuel correction value, so that the actual fuel injection is adjusted to 2.05 MS, which is close to the 1.99 ms of the normal vehicle. This indicates that the fault car is real. The air intake should also be around 2.20 g/s. Therefore, it is judged that there is an air leakage phenomenon in the defective vehicle or an electrical fault that is distorted by the air flow meter. Since the air flowmeter has been replaced before, the air leakage of the intake system is mainly investigated. After careful investigation, it was found that a vacuum tube was not inserted on the solenoid valve of the intake manifold. Troubleshooting: plug the vacuum tube, data flow recovery normal, clear fault code, test car 50 km fault light is not lit, after a period of time to return visits, users reflect all normal, complete troubleshooting. Review and summary: in the maintenance process after the failure of the car, the vacuum tube is concealed and leak, and the vacuum tube is fine, the leakage sound is very small, it is difficult to hear the engine running noise. Therefore, the maintenance personnel have not found the problem at the initial inspection, but have carried out a large amount of no one in accordance with the characteristics of the water flooded car. Specific replacement of parts. If the maintenance personnel can usually grasp the method of using data flow analysis, it is found that the air intake data is not normal, and the direction of fault detection can be determined. Even if the leakage point is more concealed, it is sure to find the root of the fault smoothly under careful investigation.