When a sensor signal is detected, the electronic control unit can only accept the sensor abnormal signal within its setting range to judge the quality of the sensor and decide whether to record the sensor. The fault code, and the sensor’s sensitivity drop due to some reason, no response and output characteristic deviation, the self-diagnosis system can not detect, of course, there is no fault code output, but the engine will display obvious fault symptoms, such as common black smoke Unstable idle speed, poor acceleration, etc. Because there is no fault code, some maintenance personnel will not know where to start or how to deal with it. During maintenance, this type of failure is difficult to judge. Under certain conditions, the fault indicator will flash. At this time, analysis and judgment should be based on the fault symptoms of the engine, and then use other diagnostic tools, such as oscilloscope, engine analyzer and oil pressure gauge. Wait for sensor target detection to find and eliminate sensor failure. For example, when the engine speed is misaligned and the unstable idling is not accompanied by the output of the fault code during driving, the air flow sensor and the intake pressure sensor have to be considered and suspected first. The performance of these two sensors directly affects the basic fuel injection quantity of the engine controlled by the ECU. Although the corresponding fault codes are not displayed at this time, they should also be checked. As another example, when the crack of the airfoil air flow meter casing leaks, it will cause the air flow meter to measure incorrectly, resulting in engine failure, but the ECU’s self-diagnosis system cannot detect this failure. .