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A BMW X5 SUV with mileage of about 185 thousand km, equipped with M62 engine and automatic transmission. User response: after starting the car, the dashboard engine fault light sometimes lights up. Troubleshooting: abnormal lighting of engine fault lights indicates fault code stored in engine control module. The connection diagnostics is self diagnostic and has a fault code associated with the two air system. Clear the fault code and perform the component test function. The two air pump is not running. Check the connection of the vacuum pipeline, normal. Check the two air pump, one-way valve, electromagnetic conversion valve and other components, found no abnormalities. Check the control circuit of the two air system, find the two air pump control relay, short the switch contact and the two air pump operation. Open the two case of the air pump relay, and find that the internal switch contacts have been burned. The relay is replaced, and the fault is completely eliminated. The function of the two air system is to purify the exhaust. The working principle is as follows: in the short time after the engine is starting, the mixture is difficult to burn completely because of the low temperature of the engine, so it will cause the discharge to exceed the standard. At this time, the fresh air is ejected to the exhaust pipe, which can make the unburned mixture continue to burn and reach the purge. The purpose of gas. The operation time of the two air pump is determined by the engine temperature and engine speed. The running time is usually 2 -60s. The two common faults of air system include one-way valve leakage, vacuum pipe crack, two air pump malfunction, two air control relay damage and so on. In the actual maintenance work, if the switch contact of two air control relays is found to be sintered, the two air pump should be checked at the same time, which is because the operation time of the two air pump is overdue after the switch contact is sintered, and it is easy to burn.