A Honda CB5 car with a mileage of over 210 thousand km and a G20A engine (five cylinder straight row engine). User response: when the car is started, both the starter and the engine are turned, but the engine is unable to function. After receiving the vehicle, the preliminary examination results showed that there was no high-pressure fire in the engine system. Check the wiring connection of the distributor, and the power cord and ground wire are good. The cylinder position sensor is installed inside the distributor. The sensor is checked and the result is normal. It is doubted that the ignition coil is defective and replaced, and the result is still in trouble. The 2 pin diagnostic block was found and two terminals were shorted for self diagnosis. The result was two fault codes 4 and 8. The meaning of fault code 4 is that the signal of the crankshaft position sensor is not good. The meaning of the fault code 8 is that the sensor signal of the TDC is not good. The crankshaft position sensor and the top dead center position sensor are mounted behind the camshaft. The two sensors are checked and the results are not normal. Replace the top dead center position sensor and the crankshaft position sensor, and the engine still can't work. Check the ignition system and pull out the high voltage line for jump fire test, resulting in no high pressure fire. Check the injector, and the fuel injector has no action when starting. Check the fuel pump. The fuel pump doesn't work when it starts. The working voltage of the fuel pump is 0V. Check air distribution phase machinery, no problem. From maintenance experience, this kind of fault is usually related to the poor connection of electrical components and lines. The top dead center position sensor and the crankshaft position sensor have been replaced. The fault code has been removed. If there is any fault code again, it is the real fault code. Once again, the engine system is self diagnosed without any fault code. There is no problem in checking the power cord and ground wire of the engine control module, and between the top dead center position sensor and the crankshaft position sensor. No fire and oil free failures are usually caused by the engine control module without the engine speed signal, but the top stop position sensor, the crankshaft position sensor, the cylinder position sensor have been checked or replaced, there is no problem, then is the fault cause for the engine control module damage? Replacing the engine control module, the problems remain complicated. After careful analysis of the cause of the failure, it is considered that the engine control module may not receive the signal of the TDC position sensor and the crankshaft position sensor correctly. Using multimeter to measure the signal voltage of the two sensors, there is almost no voltage. The oscillograph is used to measure the voltage waveform of the two sensors, and the result is very weak. The upper cover of the timing belt is removed and the installation of the position sensor of the upper and stop position and the position sensor of the crankshaft is checked. The installation position is correct and there is no loose phenomenon, but the gap with the camshaft drive pulley is too large. Is it the camshaft drive pulley installation position is not correct? So gently tapping the camshaft drive pulley, found that there is a loose phenomenon. Remove camshaft drive pulley and find it not installed. Reinstall the camshaft belt pulley, tighten the retaining bolts, start the trial run, and the engine runs smoothly. The working gap of the magnetic sensing speed sensor is usually between 0.2 and 0. 8mm, which is too large or too small to affect the signal voltage output of the sensor, even without signal voltage. Through the repair work, it is found that the work gap is not adjusted by the position sensor of the moving stop point and the position sensor of the crankshaft, the key is to correctly install the camshaft drive pulley. The inner convex point of the camshaft drive belt must be corresponding to the position sensor of the upper stop point and the crankshaft position sensor, otherwise the position sensor of the upper stop point and the position sensor of the crankshaft can not produce the correct impulse signal voltage, and the engine can not carry the car.