Driving about 29 thousand km of the 2012 Honda front. Car owner reflected: the car engine trouble light long. Fault diagnosis: connect the Honda fault diagnosis instrument after receiving the vehicle, read the fault code P0171, meaning the mixture is too thin. Clear the trouble code. After the engine has run for a period of time, the fault indicator light is lit again, the fault code is still P0171. Read the fuel adjustment data with the fault diagnostic instrument. The short-term fuel adjustment value is 1.45, indicating that the mixture is sparse. Therefore, the engine control unit wants to increase the fuel injection rate to correct the air-fuel ratio, so as to ensure that the emission reaches the standard. From the correction results, the air fuel ratio sensor signal is - 0. 01mA, the oxygen sensor signal is 0. 74V, all normal, indicating that the short-term fuel adjustment goal has been reached. However, the cumulative long-term fuel adjustment value has reached the limit of 1.25, resulting in P0171 (mixed gas dilute) fault code. Because of the basic fuel injection, the engine control unit is determined according to the engine speed and intake volume, so the engine speed and the intake system related parameters are viewed. At idle speed, the engine speed is 699r/min, normal (standard range 670r/min + 50r/min); the absolute pressure of the intake manifold is 27 kPa, normal; the intake volume is 1.2g/s, and the normal value should be around 2.0g/s. At this point, it can be judged that the intake volume of the air flow meter is low, resulting in a smaller fuel injection volume calculated by the engine control unit. But is this the result of the failure of the air flow meter itself? Further look at the idle target throttle instruction, which indicates the electronic throttle opening required by the engine control unit. Its value is 0.8 degrees, and its normal value should be about 2 degrees. It can be explained that the intake volume is small because the engine control unit initiatively reduces the electronic throttle opening. To further compare the absolute pressure data and the intake volume data of the intake manifold, the pressure value in the intake manifold is normal, but the flow of the flow through the air flow meter is small, indicating that the extra air is directly entered into the intake manifold without air flow meter detection, that is, the air leakage phenomenon. The injection volume is calculated according to the air flow passing through the air flow meter, which will inevitably lead to the excessive dilution of the mixture. The engine control unit initiatively reduces throttle opening to correct this error air-fuel ratio. Troubleshooting: after careful examination, it was found that there was a leak in the brake assisted vacuum tube. After removing the gas leakage point, test run, confirm troubleshooting and normal data.