When a TOYOTA crown (CROWN) 3.0L sedan is repaired, the detonation sensor (KNL) connection line is broken in the process of removing the cylinder head, and the repair personnel only connect the wire after it is found, and the shield line is not repaired. After the engine is repaired, only a few minutes of fault light will be run, and the fault code of the test is 55 (that is, the fault occurs). Because the car has no fault before entering the factory, the fault is caused by the repair. When the code is removed, the engine is restarted again, and the failure is still. At first, it was suspected that the KNL was damaged, so the operation voltage was measured. It was found that the normal pulsating voltage below 0.5V indicates that the KNL is working normally. When detecting the relevant components and lines, it is found that the shielding line is broken, the shielded wire is connected, and the trouble is eliminated after the code is removed and the engine is restarted. The fault of the weak electric control system of the engine is not necessarily caused by the failure of the control element, the actuator itself or the connection line between the two, and the electromagnetic interference can also cause the fault. The failure of the control system caused by electromagnetic interference occurs mainly in the operation of the engine. Once the engine is stopped, the fault phenomenon will disappear.