The earliest automotive electronic control system was the electronic fuel injection (EFI) system developed by Bosch in Germany in 1968. This system was initially used in Volkswagen passenger cars and was known as the Bosch K system. The K system is often referred to as a mechanical injection system. Idle speed and acceleration were controlled by a deflector at the throttle body that moved a linkage mechanism. This method was later improved to the KE system, used in Mercedes-Benz 124 and 126 chassis models, Audi 80 and 90 models, and some BMW vehicles. In 1974, Bosch and Volkswagen jointly launched the Bosch D system. This system achieved almost complete electronic control. It used an intake manifold pressure sensor to provide intake pressure signals and a Hall sensor to provide engine speed signals to the ECU. The ECU then used a computer to provide variable pulse times to the injectors, thereby controlling the amount of fuel injected under different operating conditions. This improved injection system was adopted by Audi, Mercedes-Benz, Volvo, Volkswagen, BMW, and other brands. In 1975, Cadillac began using a fuel injection system on some of its models, known as the Bosch L-type. This system differed from the D-type in that the L-type relied on an airflow meter to provide airflow signals to the ECU (Engine Control Unit) (the airflow meter directly generates pressure drop signals without conversion). Simultaneously, a vehicle speed sensor provided engine speed signals. The ECU processed and calculated these signals before providing variable pulse times to the injectors, controlling the fuel injection quantity. By the 1980s, cars produced in Europe, America, and Japan largely adopted fuel injection systems, each with its own unique control methods. At this time, all manufacturers of electronic fuel injection vehicles began to consider the impact of throttle load, coolant temperature, and intake air temperature on engine performance. They started using signals from sensors such as throttle position sensors, coolant temperature sensors, and intake air temperature sensors to correct the computer-calculated injection pulse times, ensuring a relatively ideal air-fuel mixture (air-fuel ratio of 14.7:1) under all operating conditions. This combination of fuel injection and airflow systems ensured both fuel economy and power in automobiles. Building upon this foundation, automakers began adopting automatic transmission control systems (TCUs), anti-lock braking systems (ABS), airbag control systems (SRS), cruise control systems (CC), anti-skid systems (ASR), and air suspension systems. They even integrated computer-controlled auxiliary systems such as air conditioning and audio systems. Particularly after the 1970s, the development of integrated circuits, large-scale integrated circuits, and very large-scale integrated circuits in the field of electronics provided automobiles with fast, powerful, reliable, and low-cost electronic control systems. These electronic control systems significantly improved the power, economy, safety, and comfort of automobiles. The widespread application of these automotive electronic technologies in the automotive industry can effectively address global issues related to vehicle emissions and the energy crisis. Therefore, the widespread and in-depth adoption of electronic technology is not only an urgent need for automakers to improve product performance and competitiveness, but also a result of government and societal support, advocacy, and even mandatory implementation. However, the electrification of vehicle control has also brought new problems. On the one hand, automotive electronic control systems are becoming increasingly complex, bringing more and more difficulties to automotive repair work and placing higher demands on automotive repair technicians. On the other hand, the safety and fault tolerance of electronic control systems are crucial; a car cannot become uncontrollable or inoperable due to sudden failures in the electronic control system itself. To address this, automotive electronic control system designers have added a fault self-diagnosis module to their designs. This module continuously monitors the operation of each component of the electronic control system during vehicle operation. If an anomaly is detected, it uses a specific algorithm to determine the specific fault, stores it in code form, and simultaneously activates the corresponding fault-handling module. This allows the faulty vehicle to be driven to a repair shop for maintenance, where repair personnel can use the self-diagnosis function to retrieve the fault codes and quickly locate and repair the problem. Therefore, from the perspective of safety and ease of maintenance, all automotive electronic control systems should be equipped with fault self-diagnosis functionality. Since General Motors first adopted fault self-diagnosis in its automotive electronic control systems in 1979, major automakers worldwide have followed suit, equipping their electronically controlled vehicles with this function. Fault self-diagnosis has become an indispensable tool for both new car manufacturing and repair shop fault detection. After decades of development, fault self-diagnosis modules can not only address the safety and memory of automotive electronic control systems and faults, but also provide real-time information on various automotive operating parameters. The basic principles and components of fault self-diagnosis: Today's computer control systems are extremely complex. Diagnosing these systems using methods previously employed by computer control would be endlessly time-consuming. Therefore, most engine computer controls have self-diagnostic capabilities. Entering a self-test mode, the computer can evaluate the operation of the entire engine electronic control system, including itself. If faults are found, they are either identified as hard faults (as needed) or intermittent faults. Each type of fault or error is assigned a digital fault code stored in the computer's memory. Hard faults refer to faults found somewhere in the system during self-testing. On the other hand, periodic faults indicate that a fault has occurred (e.g., poor contact causing periodic open or short circuits), but this fault does not appear during self-testing. Persistent RAM allows periodic faults to be stored until a certain number of ignition switch open/close cycles. If the fault does not reappear during this period, it is deleted from the computer's memory. There are many different methods for determining the fault codes generated by the computer. Most manufacturers have diagnostic instruments to monitor and test the electronic components of their vehicles. Aftermarket service companies also produce testing tools that can read and record input and output signals passed through a computer. Another method for reading fault codes is using an analog voltmeter. Some vehicles display codes via flashing dashboard lights or directly on a CRT screen. Before performing self-diagnosis or reading fault codes, perform an external inspection to determine if the fault is due to wear, loose connections, or a loose vacuum hose. Check the air filter, throttle body, or fuel injection system. Don't forget the PCV system and vacuum hoses. Make sure the evaporative carbon canister is not saturated. Inspect the wiring, connectors, charging, and AC motor systems. Also, check for signs of corrosion on the connectors. Low-voltage signals in modern circuits cannot tolerate increased resistance caused by connector corrosion. With the development of electronic technology, single-chip microcomputers, due to their significant advantages such as small size, low cost, and high reliability, have been increasingly widely used in automotive electronic control, greatly improving vehicle performance, economy, emissions control, and comfort. However, the electronic control of automobiles has brought increasing difficulties to the diagnosis and repair of vehicle faults, placing higher demands on automotive repair technicians. In this context, automotive electronic control technicians, leveraging the computer's ability to not only test and control but also easily diagnose faults using software programs, have incorporated self-diagnostic and fault-running functions into their electronic control system designs. The self-diagnostic function utilizes the ECU to monitor the operation of each component of the electronic control system. Upon detecting a fault, it automatically initiates a fault-running program, ensuring the engine can continue to operate even with a fault, and providing fault information to the driver and repair personnel, facilitating timely fault detection and resolution. The advent of self-diagnostic functions has simplified the repair of electronically controlled vehicles, making it highly popular among users. Since General Motors implemented self-diagnostics in its gasoline injection system in 1979, almost all microcomputer-based control systems in automobiles have incorporated self-diagnostic functions. The following section will introduce methods for diagnosing electronically controlled vehicle faults using self-diagnostic functions, covering aspects such as the principles and fault operation of self-diagnostics, methods for reading and clearing fault codes, and an introduction to the OBD-II diagnostic system. (I) Principles of Self-Diagnosis and Fault Operation During normal vehicle operation, the voltage values ​​of the input and output signals of the Electronic Control Unit (ECU) have a certain range of variation. When the voltage value of a certain signal exceeds this range, and this phenomenon does not disappear within a certain period of time, the ECU determines that a fault has occurred in this part. The ECU stores this fault in the form of a code in its internal random access memory (RAM) and illuminates the fault indicator lights (such as CHECK ENGINE, SRS, ABS, etc.). This is the basic principle of fault self-diagnosis. When a circuit malfunctions, its signal cannot be used as a control parameter for the engine. To maintain engine operation, the ECU retrieves a fixed value from its program memory (ROM) as an emergency parameter for the engine, ensuring that the engine can continue to run. When the microcomputer system in the ECU malfunctions, the ECU automatically activates the backup control circuit to perform simple control of the engine, allowing the car to be driven home or to a nearby repair shop for repairs. This function is called fault operation, also known as "limp" mode. On the other hand, when the ECU detects a fault in an actuator, it takes some safety measures for safety reasons. This function is called fail-safe. ECU fault diagnosis is performed on sensors, microcomputer systems, and actuators within the system. When sensors and microcomputers malfunction, fault operation mode is often adopted. When actuators malfunction, fail-safe measures are often implemented. (II) Sensor Fault Self-Diagnosis and Fault Operation Since sensors themselves generate electrical signals, fault diagnosis of sensors does not require dedicated circuitry. Instead, it only requires programming a sensor input signal recognition program in the software to achieve sensor fault diagnosis. The normal input voltage value of the water temperature sensor is 0.3-4.7V, corresponding to an engine coolant temperature of -30-120℃. Therefore, if the voltage signal detected by the ECU exceeds this range, and it is only an occasional occurrence, the ECU's diagnostic program will not consider it a fault. However, if the abnormal signal persists for a period of time, the diagnostic program will determine that the coolant temperature sensor or its circuitry is faulty. The ECU stores this situation in the form of a code (this code is a pre-defined digital code representing an abnormal fault in the water temperature sensor signal) in the random access memory. At the same time, by checking the engine warning light "CHECK ENGINE," the driver and maintenance personnel are notified that a fault has occurred in the engine electronic control system. When the ECU detects that the water temperature sensor is malfunctioning, it uses a pre-set constant as a substitute value for the water temperature signal to keep the system running. (III) Self-diagnosis and backup circuit of microcomputer system If the microcomputer system malfunctions, the control program cannot run normally, and the microcomputer is in an abnormal working state. This will cause the car to be unable to drive due to the engine control system failure. In order to ensure that the car can continue to run when the microcomputer malfunctions, a backup circuit (backup integrated circuit system) is designed in the control system engineering. When the microcomputer in the ECU malfunctions, the ECU automatically calls the backup circuit to complete the control task, enters the simple control operation state, and uses a fixed control signal to keep the vehicle running. Since this system only has the simple function of maintaining engine operation and cannot replace all the work of the microcomputer, the operation of this backup circuit is also called "limp" mode. When the backup system is working, the fault indicator light is on. Whether the microcomputer is working normally is monitored by a circuit called the monitoring circuit. The monitoring circuit has a counter that is independent of the microcomputer system. When the microcomputer is running normally, the microcomputer's running program resets the counter periodically. In this way, the value of the counter in the monitoring circuit will never overflow. When the microcomputer system malfunctions, the microcomputer cannot reset the counter at regular intervals, causing the monitoring counter to overflow. When the monitoring counter overflows, the output level changes from low to high (this output is generally the carry flag of the counter. When the counter reaches its maximum value, another counting pulse is added, and the counter overflows. At this time, the level of the overflow terminal of the counter will change from low to high; at the same time, the counter is reset to zero). This change in the counter output level will directly trigger the backup circuit. The backup circuit only controls the injector and igniter with a constant injection duration and ignition advance angle according to the start signal and the idle contact closed state. (IV) Fault diagnosis and fault insurance of actuators In the automotive electronic control system, the actuator is the main device that determines the engine operation and vehicle driving safety. When the actuator fails, it will often have a certain impact on the driving of the vehicle. Therefore, the typical handling method for actuator failures is as follows: when an actuator failure is confirmed, the ECU takes appropriate safety measures based on the severity of the failure. A fault-prevention system is also specifically designed within the control system. Since the ECU performs control operations on the actuators, the control signal is an output signal. Therefore, to diagnose the operation of each actuator, a fault diagnosis circuit is generally added. That is, the ECU sends a control signal to the actuator, and the actuator has a dedicated circuit to feed back its execution status to the ECU. In the engine electronic control system, the ignition coil is a typical component for actuator fault diagnosis. Under normal circumstances, when the ECU controls the ignition coil, each time the ignition coil performs ignition, the ignition confirmation circuit within the ignition coil feeds back the ignition execution status to the ECU in the form of an electrical signal. When the ignition circuit or ignition coil malfunctions, the ECU sends an ignition control command but receives no feedback signal; at this time, the ECU considers the ignition coil to be malfunctioning. Because if the ignition system malfunctions during engine operation, unburned air-fuel mixture will enter the exhaust system and exhaust pipes. The catalyst temperature in the exhaust purification device will greatly exceed the allowable value. Simultaneously, excessive accumulation of unburned air-fuel mixture in the exhaust pipe can cause an explosion in the exhaust system. Therefore, a fail-safe system is employed; when the ECU fails to receive an ignition confirmation signal, it immediately cuts off power to the fuel injection system, stopping fuel injection.