FAW TOYOTA corolla car. The car was repaired in a garage due to a traffic accident. After the repair of damaged parts, the test found that the engine was idle, the exhaust pipe was slightly smoky, and the fuel consumption increased. But the engine is all right in the middle and high speed. Fault diagnosis: in order to solve this problem, the repair plant once replaced idle control valve, air flow meter and two oxygen sensors, cleaned the oil road several times, and checked the related lines and plugs many times. However, the fault was not solved. After the 4S shop technician took over the car, the computer inspecting instrument was first used to check it. The diagnostic instrument displayed no trouble code storage. Because the diagnostic instrument used is not a factory special detector and can not check the dynamic data of the engine, a basic routine check is carried out and 4 spark plugs are removed. The head of the spark plug is found black. According to the black smoke of the car and the black carbon in the head of the spark plug, it is preliminarily analyzed that the mixture of the car is too mixed. . The reasons for the overconcentration of mixed gas are too high fuel pressure, stagnation or leakage of the injector, excessive intake pressure, weak or lack of high pressure spark, electrical control system or engine mechanical failure. After understanding, the car did not have any trouble before the accident occurred, and it did not repair the engine in the process of rice gold repair. There is no damage to the electric control part of the original car line. Then check the intake system, check the throttle body, the idle control valve and the related vacuum pipeline carefully to see if there are clogged or dirty places affecting the mixture of gas too thick and cause failure, but no anomalies are found. Then the cylinder pressure, fuel pressure and high voltage spark of the 4 ignition coils were measured, which are all normal. The 4 injectors have no oil leakage and hairpin phenomenon, and the atomization is good. The active carbon tank solenoid valve works well. Using an oscilloscope to test the signal of the former oxygen sensor, it is found that the signal voltage is low and the clutter is too much. At that time, the engine mixed gas was actually too thick. If the oxygen sensor was working normally, the high voltage signal should be produced. The oxygen sensor of the vehicle is new, and the sensor itself will not have any problems. If the oxygen sensor itself has good performance, this contradiction will arise. The maintenance personnel consider only one situation, that is, the oxygen sensor detects the wrong exhaust gas information, one is "pseudo dilute" phenomenon, and the "pseudo dilute" signal usually occurs in the following two cases: (1) the "pseudo dilute" caused by the spark plug or a few ignition coils, when the individual spark plug can not be ignited normally in the cylinder. The unburned gasoline and air are discharged into the exhaust pipe, and the oxygen sensor detects that the mixture is too thin, because the oxygen sensor is only a sensor to monitor the oxygen content, and whether the oxygen is burned, the sensor does not distinguish it, and the oxygen entering the exhaust pipe will produce "false dilute" phenomenon. 2. If the exhaust manifold leaks, if leakage occurs between the exhaust valve and the oxygen sensor, it will cause "pseudo dilute". Because when an exhaust pulse passes through the air leakage area, a low pressure area is formed behind the pulse, which will inhale the air outside, enter the exhaust air of the exhaust pipe, and pass through the oxygen sensor, when the oxygen entering the exhaust gas drops the oxygen sensor voltage, thus the control unit considers the mixture to be mixed. It is necessary to increase the amount of oil supply when the gas is too thin. According to the above test and analysis, the second situations are more likely. Start the engine carefully and listen carefully. The engine exhaust pipe has a slight leak. After dismantling the exhaust pipe, it is found that the exhaust pipe interface pad has the shift marks in the accident. The interface pad has been damaged, the exhaust pipe interface pad and the 4 spark plugs are replaced. Considering the long effect fuel correction function of the control single element, the control system has been controlled. The unit can not be properly corrected, so the engine control unit must be restudied, and then the vehicle is circulate several work to let the engine run in various working conditions, and the effect is good. After the trial run, the symptom of the vehicle failure has disappeared. The oxygen sensor signal and waveform display have returned to normal.