A Volkswagen Teng 1. 8TSI manual car with a mileage of about 153 thousand km in 2008. The user reflected that the car had occasional setbacks during its journey, and was unable to accelerate. After receiving the car: check the car with the fault diagnosis instrument and read the fault code 08851 P2293. The fault code is defined as the mechanical fault of the fuel pressure regulating valve N276. According to the previous maintenance experience, the author determines that the possible causes of the fault code are: low oil pressure of low pressure oil pump, serious leakage of pressure, problems in the related circuit, damage of N276 itself, mechanical failure of high pressure oil pump itself or problem of engine control unit. Then the author uses the fault diagnosis instrument to read 140 sets of data flow in the idle speed of the vehicle. The display group shows the normal data as shown in Figure 1, indicating that the fuel pressure regulating valve N276 and the related circuit are normal. After deleting the fault code, the fault code is reproduced. Since the circuit is normal, the fault lies in the oil circuit. The author progressively inspected the oil road. When checking the low pressure oil pump, it is found that a large number of impurities are blocked up in the oil pump screen. Troubleshooting: use the carburetor cleaner to clean the filter screen and replace the gasoline filter element and install the trial run. The failure disappeared. To eliminate the trouble thoroughly, the fuel low pressure oil pump is replaced after the user agrees, and the fuel tank and the oil circuit are cleaned. After the vehicle was delivered, after a period of return visit, the fault did not appear again. At this point, the fault is completely eliminated. The engine control unit calculates the fuel injection according to the air flow meter, speed, engine speed, throttle opening and acceleration pedal position, and controls the oil supply of low pressure oil pump, high pressure oil pump and injector. At the same time, the high pressure sensor signal is used to monitor the working condition of the high pressure oil pump. When the signal detected by the high pressure sensor exceeds the control range, the engine control unit will consider the N276 failure of the fuel pressure regulating valve no matter how high or too low the oil pressure. This car is due to the low pressure oil pump screen is blocked by impurities, resulting in low pressure oil pump to high pressure oil pump fuel shortage, resulting in too low fuel pressure cause failure. However, because there is no low-voltage sensor in the car, it can not directly monitor the working condition of the low-pressure oil pump. Although the fault code stored in the engine control unit is the fuel pressure regulator N276 fault, the cause of the failure is not generated by N276. If the replacement parts are simply based on the fault code, it will not only be able to eliminate the fault, but also make the users suffer economic losses. Therefore, for similar failures, the maintenance personnel are required to know the structure and working principle of each system in order to determine the real location of the fault point.