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A Dongfeng Citroen dual fuel taxi in 2011 with a mileage of over 280 thousand km. The owner reflected that the car air conditioner often shut down automatically and repressed the failure that the switch did not respond. Fault diagnosis: it is found that once the air conditioner is shut down automatically, there will also be problems such as high speed operation of the fan motor, the STOP light on the combined instrument, the light of the engine fault alarm, the short information of the multi-function display display, and the use of the diagnosis computer to the engine computer, and the second cylinder fire in the engine computer. Failure. According to the information on the maintenance manual of the triumph car, the symptoms caused by the failure of the engine temperature sensor are exactly the same as that of the triumphant taxi car. So, 4S store directly changed the water temperature sensor, but after the trial, the fault remained. The electronic control system is composed of sensors, computers, and actuators. Since the replacement of the sensor is invalid, will it be a problem of the wiring harness between the computer or the sensor and the computer? Check the wiring harness between the temperature sensor and the computer, normal. Then try to change the computer and remove the computer and the intelligent control box BSI on the same type of car, and install it on the fault car. As a result, when the fan is high, the fault lights go on, the air conditioning is closed automatically, and the maintenance is in a deadlock. According to the analysis of the repair plant, it may be the problem of the gas burning system, so the tank is filled with gasoline, and the fault is still in the car after the test. After analyzing the situation of the vehicle, the author should check the high temperature of the engine since it is caused by high temperature. There are only two reasons for the high temperature of the engine, one is excessive heat, the other two is insufficient heat dissipation. The author checked the cooling system no problem, and endoscopy examination, found that the engine carbon deposition too much (carbon deposition too much affect heat dissipation). Lenovo to the cylinder has a fire failure, so focus on the inspection of the intake gate pole, it is sure to accumulate a lot of carbon, the decision has caused the valve to close the tight, cause the cylinder fire, clean up clean carbon test, troubleshooting. Maintenance summary: in view of the fact that dual fuel taxis mainly burn natural gas, there will not be so much carbon deposition. But the dual fuel car needs to start with gasoline, and the number of taxi flameout, frequent use of cars, fuel conversion, flameout, excess gasoline at startup, excessive mixture of gas, walking and stopping road conditions, will cause excessive carbon accumulation. If we do not pay attention to regular cleaning, once the carbon is wrapped into the valve pole, the valve closes are not strict, it will inevitably cause the failure of the fire, and the high temperature will cause the valve deformation. Once the deformation is made, the removal of carbon can not be eliminated, and the grinding valve and even the cylinder cover are needed. The engine can not work properly under the downgraded limp mode. This also shows that the reason for the air conditioning fault is not only in the air conditioning system itself.