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The mileage is about 18 thousand km, which carries the Chevrolet 1.8L Cruz car in 2009. The car was overhauled because of engine dithering and engine fault lights. Fault diagnosis: connect the GDS to the fault code, and detect the engine control module (K20) memory with the fault code "P0304 detects the cylinder 4 non ignition", and is the current fault. So the engine idling is caused by a fourth cylinder fire. The analysis shows that the cause of the fault may be as follows: spark plug fault, ignition coil fault, injector fault, low cylinder pressure, engine control module fault, related line fault and so on. In line with the principle of simple to propagation, the maintenance personnel interchanged the spark plug and the ignition coil with the fault car and the same type of normal vehicle first, but the fault remained, indicating that the ignition coil and the spark plug should be normal, and then the spark plug and the ignition coil of the original car were reassembled. Check the control line of the fourth cylinder ignition coil. According to the circuit diagram, check the line between the terminal 16 of the wire connector X1 of K20 and the terminal G of the ignition coil wire connector T8. The line is good, and there is no short circuit or circuit breakage. The cylinder pressure gauge is connected to measure the cylinder pressure of 4 cylinders respectively, which are all about 1180 kPa, and the cylinder pressure is normal. Since the ignition system and the cylinder pressure are not found abnormal, the maintenance personnel decide to check the injector, remove the injector and install it on the test machine of the injector, and the injector has a uniform injection and good atomization. Then check the control line of the injector, according to the circuit diagram, measure the voltage of the wire connector terminal A of the fourth cylinder injector, 12.69 V, normal; check the line between the terminal B of the injector wire connector terminal B and the terminal 34 of the K20 wire connector X1, with good conductance, no short road, circuit breakage and so on. According to the above inspection results, it is speculated that the internal damage of K20 caused the fourth cylinder injector not to spray fuel. Replacement of K20 and commissioning after commissioning, the fourth cylinder injector still does not spray. At this point, the maintenance is in a deadlock. Suspicious fault points have been investigated. What causes the failure? Taking into account that the fault code is still P0304, the maintenance personnel try to replace the ignition coil again after trial run, troubleshooting. Maintenance summary: the ignition coil and K20 of the car have faults (suspected that the internal fault of the ignition coil causes K20 damage). At first, the possibility of eliminating the fault of the ignition coil and spark plug with replacement method is not fully considered, which makes the troubleshooting process a bit curved. After the replacement of K20, due to the failure of the ignition coil still not ruled out, K20 still monitored the fourth cylinders not ignition. Out of emission control and safety design factors, K20 continued to control the fourth cylinder injector not to fuel oil.