Audi A6 1.8LMT car, which runs about 110 thousand km. The tail gas of the car has a peculiar smell and has a black smoke. Fault diagnosis: 1) first, use V.A.G 1552 to enter the 01-02, query the engine self diagnosis system and have 1 fault codes 00561, which means that the mixing gas adjustment exceeds the limit. 2) as the vehicle smoked black, it looked at the data related to the oxygen sensor. When V.A.G 1552 enters 01-08-007, the one or two zone data are 25% and 0.815V respectively when the engine is idle. The second is the voltage of the oxygen sensor, and its value should change with the concentration of the mixture from 0.2 to 0.8V, and the voltage of the mixed gas is 0.2 to 0.4V, and the concentration of gas mixture is 0.6 ~ 0.8V. 3) the oxygen sensor of the vehicle is 0.815V, indicating that the mixture is too thick, which coincides with the fact that black smoke is emitted. The control value of mixed gas is formed in one area. The normal value is - 10% - 10%, and it fluctuates slightly with the correction of the amount of fuel injection by oxygen sensor. The car is - 25% to reduce the amount of fuel injection. 4) why does oxygen sensor decide that the mixture is too thick, but does not reduce the injection volume to form suitable mixture? The injection time of EFI engine is mainly determined by engine intake and engine speed, while oxygen sensor only modifies fuel injection within a certain range. 5) when the vehicle is idle, the oxygen sensor reduces the fuel injection volume by 25%, which is far more than the standard value of 10% to 10%. Because the oxygen sensor has reached the limit, the mixture is too strong, so the engine records 00561 fault codes, and the problem concentrates on the air flow meter that determines the amount of fuel. 6) use V.A.G 1552 to enter 01-08-002, and the data is shown in Table 1. The intake volume of the four zone is 7.6 to 7.8g/s, while the standard value is 2 to 4.0g/s at idle speed. The two area is the theoretical injection time within a week of crankshaft, and the three area is the actual injection time of an engine cycle. 7) the actual injection time is revised on the basis of theoretical injection time, about 2 times of theoretical injection time, but the vehicle is almost the same. This proves that the air flow signal is too large and the theoretical injection time is longer. After correction, the actual injection time is obviously shorter, but because the correction exceeds the limit, no suitable mixture can be formed. 8) in order to further prove that the signal of the air flow meter is too large and read the data of 2 normal vehicles, see Table 1. 9) the mixture is too thick, but why is the fuel consumption normal? Again 01-08-007, when the engine is idle, the one or two zone is - 25%, 0.815V, and after the acceleration pedal, the value changes to 2% ~ 3.7%, 0.2 ~ 0.8V, which indicates that in other conditions, the mixture gas is normal. In general, the running time of vehicles in idle condition is relatively short, and the mixture is too thick at idle speed, which has little effect on fuel consumption. 10) after the air flow meter is replaced, the fault is eliminated.