1. The method of removing the battery cable can not be used to clear the fault code. For most electronically controlled engines, remove the battery cable or remove the fuse leading to the ECU and keep the power off for 30 seconds to clear the stored in the ECU. error code. But for some engines, this method is not suitable, because the memory of vehicle anti-theft, audio, quartz clock, etc. are also stored in the random access memory, and these memories will be cleared together after power off, which will cause audio lock codes and other phenomena. For these engines, the fault code should be cleared according to the method required in the maintenance manual, and the battery connection line must not be removed at will. 2. The battery connection line cannot be removed before reading the fault code recorded by the engine electronic control unit because the battery connection line or the power fuse is rashly removed before reading the fault code, and the power supply of the ECU is interrupted and stored in its random access memory. The fault code in will be automatically eliminated. If you want to get the fault information again, you must repeat or reproduce the working state and environmental conditions when the fault occurs. For the engine that cannot be started, the fault code can no longer be obtained after the operation, and an important fault judgment information is lost. Therefore, Before repairing the electronic control engine, the fault code recorded by the ECU should be performed as required before other repairs. 3. When the ignition switch is in the on position, the battery cable cannot be removed casually. When the ignition switch is in the position, no matter whether the engine is running or not, never remove the battery cable or unplug the power fuse. Because a sudden power failure will cause the coils in the circuit to generate self-induced electromotive force, resulting in a very high instantaneous voltage, which will seriously damage the ECU and related sensors and other microelectronic devices. In addition, any wires of other electrical devices with the same voltage as the battery cannot be removed when the ignition switch is in the ON position, otherwise the relevant coils will also generate self-induced electromotive force, which will then be used by the ECU or sensor. These electrical devices include: ignition system, idling stepper motor, programmable read-only memory of ECU, fuel injector, air conditioner and other electromagnetic clutches, as well as some connecting wires of ECU.