Fault detection and maintenance diagnosis of automobile engine electronic control system refers to the process of detecting one or several fault symptoms by certain means and making correct judgments. Comprehensive diagnosis technology refers to the whole process of using all possible and necessary detection means to detect complex fault symptoms, and carefully analyzing the detection results (including various data parameters) from the surface to the inside, from the shallow to the deep, eliminating the false and retaining the true, so as to get the most practical judgment, and constantly verifying and correcting the original judgment in further disassembly and repair until the fault is truly eliminated. It usually includes the following parts: (1) fault code analysis; (2) data analysis (including waveform analysis); (3) Ignition analysis (including waveform analysis); (4) Tail gas analysis (including waveform analysis); (5) Pressure and vacuum analysis (including waveform analysis). Fault code analysis is a method to compare and analyze the read fault codes and make fault judgment on the basis of reading fault codes and combining with other test results. It is one of the most basic and simple methods in fault diagnosis of automobile electronic control system. The process of fault code analysis is to read, clear, identify and classify the fault codes recorded by the fault self-diagnosis system of automobile control computer. Usually, fault code analysis is the first step to diagnose the fault of automobile electronic control system. Fault code (fault code for short) is the corresponding code (number or letter) recorded by the self-diagnosis system of automobile control computer for the detected fault point. According to the different display methods of data on the detector, data parameters can be divided into numerical parameters and state parameters. Data parameters are parameters with a certain unit and a certain range of change, which usually reflect the working voltage, pressure, temperature, time and speed of each component of the electronic control device. State parameters are parameters with only two working states, such as on or off, on or off, high or low, yes or no, etc. They usually indicate the working state of switches and solenoid valves in electronic control devices. According to the control principle of ECU, data parameters are divided into input parameters and output parameters. Input parameters refer to the parameters input to ECU through sensors or switch signals. Input parameters can be numerical parameters or state parameters. The output parameter is the output instruction sent by ECU to each actuator. Most of the output parameters are state parameters, and a few are numerical parameters. The parameters in the data stream can be classified according to each system of automobile and engine, and the parameter analysis methods of different types or systems are different. In the fault diagnosis of electronic control devices, the parameters of several different types or different systems should also be comprehensively compared and analyzed. The names and contents of data flow parameters of electronic control devices of different brands and models of automobiles are not exactly the same. Data parameter analysis is one of the important methods for fault diagnosis of electronic control system. Data parameters are the quantitative expression of the control state of the controlled system by the control computer. Data parameter analysis is a process of comprehensive analysis of various related data parameters of control system by using various test methods. Data parameter analysis is divided into two ways: numerical display and waveform display, computer communication measurement, circuit on-line measurement and component simulation measurement. When analyzing some data parameters, the computer should not only consider the value of the sensor, but also judge its response speed in order to obtain the best control effect. For example, the signal of oxygen sensor requires not only the signal voltage and voltage change, but also the frequency of signal voltage change in a certain period of time (for example, some cars require more than 6 ~ 10 times/10). When it is less than this value, a fault code will be generated, indicating that the response of the oxygen sensor is too slow. Faults with fault codes are relatively easy to solve. However, when the number of times does not exceed the limited value and the response is already slow, no fault code will be generated. At this time, if you understand it carefully, you may feel some fault symptoms. We should connect the instrument to observe the data of the oxygen sensor (including the signal voltage and the change state of about 0.45V to judge the quality of the sensor). For example, in an Audi car, when the oxygen sensor is slow to respond, the rotation speed often automatically fluctuates between 1600 ~ 1800 r/min (when the accelerator pedal is not moving), which even affects the acceleration. This is often due to the slow response of the oxygen sensor, which leads to the excessive change of air-fuel ratio and the fluctuation of speed. Similarly, for vehicles equipped with OBD-ⅱ system, the signal change frequency of oxygen sensor before and after catalytic converter is different. In general, the signal change frequency of the rear oxygen sensor should be at least half that of the front oxygen sensor, otherwise the conversion efficiency of the catalytic converter may be reduced. For example, the oil pressure alarm system of Audi car uses high and low pressure alarm. When idling is specified, the alarm will be given when the pressure at the low pressure sensor (generally installed at the rear side of the cylinder head) is less than 30kPa, and when the pressure at the main oil circuit (sensor installed at the machine filter) is less than 180kPa. When the car is idling, it will give out a high-pressure alarm. After inspection, the speed signal is wrong. After replacing the ignition module, the system is normal. Because the alarm control system obtains the rotation speed signal from the ignition module, the actual rotation speed is (800 50) r/min at idle speed, while the rotation speed signal obtained by the alarm system is close to 2000 r/min, but the oil pressure at this time will not reach 180kPa or more, and an alarm will naturally be given. When there is a fault code, when the fault code is confirmed by analysis, we can directly find out all groups of data related to the fault code for analysis, analyze the causes of the fault code according to the conditions set by the fault code, and then analyze the numerical value and waveform of the data to find out the fault point.