TOYOTA's Costa passenger car with a mileage of over 210 thousand km in 2010. User response: the car is powerless and engine trouble light is on. Inspection and analysis: the maintenance personnel detected the engine control unit and found that the fault code was P0171 - the mixture was too thin. The engine control unit determines the basic injection time according to the engine speed, air flow and air inlet pressure, and corrects the basic injection time according to the numerical value of the air fuel ratio sensor, so that the mixture is controlled near the theoretical air fuel ratio 14.7: 1. In this way, the performance of the engine can not only be brought into full play, but also the transformation of the three way catalytic converter is optimized. When the mixture is too dilute, the engine control unit will increase the injection volume. If the injection volume is increased by more than 35%, the fault code P0171 will appear. According to the principle, the possible fault locations include intake system, exhaust system, fuel injector, fuel supply system, engine control unit and air-fuel ratio sensor. Check the set data, speed 41 km / h, engine speed 41 / h, engine speed 1604 r/min, air flow 22.5, atmospheric pressure 71 kPa, coolant temperature 840C, air fuel ratio sensor signal 3.18 V, post oxygen sensor signal 0.8 V, short-term fuel correction 17.9%, long-term fuel correction 25%. The total fuel oil is corrected to 38.2%, abnormal. Replace air flow meter, the failure is still. No obvious air leakage was found in the intake system check. Remove the throttle check, the sealing ring is good. No problems were found in the exhaust system, fuel supply system, engine cooling system, PVC valve and connecting hose. The air-fuel ratio is actively tested, the mixture concentration is 12.5%, and the reduction is 12.5%. The test results show that the air fuel ratio sensor and the oxygen sensor are normal. The 4 injectors are cleaned, and the fault is still unchanged. Check the carbon tank and the carbon tank solenoid valve, all normal. Replace the engine control unit, the failure is still. Again, check the air intake system, or failed to find the gas leak, but unexpectedly found that the new air filter is unknown brand, the original problem is here. Troubleshooting: data flow is back to normal after replacing the brand air filter. The short-term fuel correction is about 0.7%, the long-term fuel correction is about 3.9%, and the total fuel is revised to 6.2%. The atmospheric pressure is 101 kPa, and the fault is ruled out.