A FAW Volkswagen Sagitar 1.6L car with a mileage of about 80 thousand km and a manual transmission was installed in 2006. Owners reflect: the car usually pays attention to maintenance, the vehicle performance is good, there is nothing unusual, but suddenly once the engine can not start at work. Fault analysis: the car starter turns strong, but there is no sign of ignition, so it can be determined that the starting system and the power supply are all normal. The problem may be in the engine ignition system, the oil supply system, the mechanical part, the exhaust system and the engine control system. Fault diagnosis: first, the public special diagnostic instrument V.A.S5052A is connected to read the fault code of the vehicle electronic control system, but the fault code is not found; at the same time, the static data flow of the engine is read, and there is no exception. Then check the ignition system of the engine: pull out the high pressure ignition coil of each cylinder, install the spark plug, carry on the fire test, find that the spark plug is normal, indicating that the ignition circuit is no problem; then 4 spark plugs are removed to check, the spark plug electrode has no obvious ablation, the clearance is normal, some carbon deposits are cleaned up. Post the spark plug. Check the engine fuel supply system: first measure the fuel pressure, pressure the fuel line, connect the fuel pressure test meter, start the engine, test the fuel pressure is 3.7bar, the pressure value is in the normal range, indicating that the electric fuel pump and its control circuit are no problem; check the injector line, pull the connector of the 1 cylinder injector, There are 1 diode test lights on the plug, starting the engine, the test lamp can flicker and light, indicating that the control circuit of the injector is no problem; it is also normal to use the multimeter to measure the damper in the injector. Check engine mechanical part: measure cylinder pressure. Dismantle the spark plug, install the cylinder pressure meter, starting engine measured 4 cylinder pressure are: 11.2bar, 11.3bar, 11.3bar, 11.4bar, cylinder pressure value is normal, indicating the cylinder seal performance is good, engine mechanical part of no problem. Through the investigation of the above systems, no problems were found. Finally, the components are reinstalled, the engine is tried again, and the accelerator is added. After a long time start, the engine can not start normally, but there are occasional signs of ignition. The spark plug is removed again. It is found that the front end of the spark plug is not wet, indicating that the fuel intake into the cylinder is too small. Therefore, when the engine is started, the carburetor cleaning agent is injected, and the engine can carry the vehicle, thereby judging that the mixture is too dilute to cause the engine to fail to start. In order to verify and judge, the V.A.S5051 oscilloscope is used to detect the injection pulse width of the injector. It can be seen that the injection pulse width is obviously too short and the mixture is too thin, which causes the engine to be unable to start normally. The injection pulse width of a normal vehicle, as shown in Figure 2. The basic pulse width of an engine depends on the speed and load of the engine. It is normal to detect engine speed sensor and intake pressure sensor further, so replace engine control unit and troubleshooting. Fault summary: in the process of no code fault diagnosis, it is difficult to judge whether the engine control unit itself has a problem. It can judge whether it is normal by the easy to judge the sensor and the actuator. When testing the fuel injection system, it is better to use an oscilloscope to analyze the fuel injection pulse width at a glance and avoid using the test lamp to make simple judgement, resulting in misjudgement.