Chery's Fengyun sedan, which has a mileage of over 276 thousand km in 2002. User response: the car will smoke black when the engine accelerates and fuel consumption will be high. Fault diagnosis: after commissioning, the fault is exactly as described by the user. The main causes of this fault include too high pressure of fuel system or injector leakage; intake pressure sensor failure or vacuum tube blockage; PCV valve card; intake system blockage or inflow; the internal component work of the engine control unit is unstable; oxygen sensor failure or bad work; cylinder pressure overload. Resulting in incomplete combustion, aging, short circuit or open circuit of related lines, resulting in distortion of sensor or actuator signal. Field inspection found that when the car accelerated, the exhaust pipe emitted thick black smoke. Connect the fault diagnosis instrument to detect the engine control system, and the device hints that the system is equipped with the relevant fault code of the oxygen sensor. The user reported that the vehicle had changed almost all electronic control system components at other service stations. Only the engine control unit has not been replaced, so the maintenance personnel suspect that the engine control unit is not working properly. On site check throttle body found that throttle body into the intake manifold fuel more. And as long as the ventilation port of the PCV valve is blocked, the engine will immediately turn off. This situation indicates that the idle speed control valve of the vehicle has been jammed. The engine keeps the idling condition from the intake of the PCV valve, and the PCV valve should not go through the atmosphere directly when it is working normally. This also indicates that the PCV valve is jammed. Due to the abnormal operation of the 2 components, the air-fuel ratio control of the engine control unit is out of order. After replacing the throttle body and the PCV valve, the test car found that the engine idle speed was smooth and the PCV valve was disconnected. The effect of the blockage on the engine was very small, indicating that the fault was correctly judged. However, there is still black smoke coming out during the repeated acceleration. Our analysis may be carbon deposits left in the exhaust pipe. After the vehicle runs 20km. Once again, the exhaust pipe is still emitting black smoke when it is again accelerated. According to the specific symptoms of the vehicle, maintenance personnel analysis may be poor oxygen sensor. But the user reflected that the car had just changed the oxygen sensor at another service station two days ago. In order to get the bottom, the maintenance staff decided to remove the oxygen sensor and check it. The inspection found that the last maintenance personnel mistakenly installed the oxygen sensor of the multi point injection system of the Mariley engine control system on the vehicle. After the correct oxygen sensor is replaced, the test run is normal and the trouble is completely eliminated. Maintenance summary: the car is a comprehensive fault, all other service stations do not exclude, because they suspect that what parts to try to change, if not, then dismantling, and then try to change other suspicious parts. It also mistakenly installed the oxygen sensor of Mariley multi point injection control system into a single point injection system, resulting in a system mismatch fault. This also reminds us that we must think more in the maintenance of the vehicle, and be good at considering the correlation and influence between the parts and components so that the failure can be eliminated smoothly and quickly.