The mileage is about 43 thousand km, and the Volkswagen Passat new 2.0L is equipped with the BNL engine. The car has been repaired many times on the light of the light. Fault diagnosis: read the fault code P0411, two air system, the flow is incorrect. After checking out the two air pump damage and replacing the new two air pump, the technician checked the lights again. After testing, the fault code is still the same fault code, and the maintenance technician is seeking advice from the author. In order to solve the fault thoroughly, we must first understand the structure and basic working principle of the two air system. The two air system structure of the car is simple. After the engine cold start (coolant temperature + 5 ~ + 3), the engine control unit is connected to the two air pump relay J299 circuit, and the two air is used. The pump is operated by electricity, and the fresh air is blown to the rear of the exhaust valve through the combination valve, so oxygen is added to the exhaust gas when the cold car starts, and the exhaust gas is reburning again, thus reducing the heating stage of the three element catalyst and reducing the emission of the harmful gas in the exhaust gas, and the engine control unit controls the J299. The longest working time is no more than 100s. In addition, when the engine is started at other times (the engine temperature is up to 96 degrees centigrade), the two air systems will also be connected to 10s at idle speed, and are tested by self diagnosis. At this point, the oxygen sensor adjustment device must be in the starting state. After understanding the relevant principles, we will analyze the problem through fault codes. The two air flow is not correct. As the name suggests, the air flow of the two air system is too little or too much, beyond the scope approved by the engine control unit, so the engine control unit reports the fault code. However, this way of thinking will naturally think of how the control unit can detect the two air flow. A sensor should be used to detect the flow of two air, but the original plant's circuit diagram and the factory's maintenance manual did not detect two air flow sensors on the engine system. This further raises the author's question. What channel does the engine control unit detect the two air flow? What is the reason to determine whether the two air flow is correct? Check the pipeline of the two air system carefully and find the inlet of the two air pump to the bottom of the air filter. It shows that the intake of the two air system is not only filtered through the air filter, but also through the calculation of the air flow sensor. In this way, the engine control unit is understood. Measure the flow of the two air. First, the intake signal measured by the air flow sensor is closely related to the engine speed load and the throttle opening. When the engine system works normally, the air flow sensor signal is the standard signal (called X). The standard signal is in accordance with the various working condition curves corresponding to the inner part of the engine control unit. Deviating from the curve range, the engine control unit considers the system malfunction, which is the premise for the control unit to memorizing the fault code and lighting the discharge lamp. In this case, when the conditions of the two air pump activation are satisfied, the engine control unit is connected to the two air pump relay J299, the two air pump works, the fresh air flowing through the air filter is pumped into the rear of the exhaust valve, and the air flow rate of the two air pump is assumed to be Y, then the flow of the air flow sensor will naturally be the flow of the air flow sensor. X+Y=Z. At this time, the Z should be kissed to incorporate the range of internal curves that conform to the engine control unit. If Z deviates from the range of the graph, the control unit will remember the related fault codes. According to the analysis of these principles, the author determines that the possible causes of this problem are as follows: (1) the fault of the sensor signal X is caused by the fault of the air flow sensor itself, which causes the X to go beyond the normal range of the engine control unit, and the control unit will remember the fault. However, if the hypothesis is set up, the abnormal X signal of the air flow sensor will be accompanied by all the operating range of the engine, and should not be limited to the working time of the two air pump. After all, the two time air pump work is very short, so it should also report fault codes such as excessive air flow or small signal. So the reason should be excluded. (2) leakage in the pipeline of the two air system. The intake pipe of the two air pump is very short, the direct one pipe is under the air filter, so the possibility of the leakage is not very possible, and the most possibility is the two air pump outgassing pipeline. The pipeline is finally connected to the mechanical valve inlet A, through the exhaust manifold, the pipeline is long, the working environment is bad, the pipeline is out. The probability of the present problem is larger. Therefore, I asked the maintenance technician to dismantle the pipeline and carefully inspect every part of the pipeline without finding any problems in the pipeline. At this point, the possibility of pipeline leakage can be ruled out. (3) mechanical valve failure. As the mechanical valve is connected to the rear of the exhaust door, it is directly connected with the high temperature exhaust, so the valve can not open the coke card and can not open and so on. In this idea, the author let the maintenance personnel dismantle the mechanical valves and let the maintenance personnel clean them carefully, and then blow air on the imported A of the mechanical valve with the mouth bar. Found the mechanical valve outlet B air flow out, at this point I think the fault should be excluded, so that the customer first drive back, try to observe the effect for a period of time. The customer calls again after a week or so, which reflects that the engine exhaust lamp is lit again. After the appointment, the fault code is read again, which is exactly the same as the last fault code. This indicates that the cause of the last failure has not been found and must be redone again. This time, I also feel very strange that the main parts of the two air system are not a problem, the two air pump is new, and what are the problems to follow? Is this fault related to the engine control unit? But it's not very likely. Why? First of all, when the customer first starts every morning, it does hear the noise of the two air pump work, which means that the engine control unit can control the normal work of the two air pump. Next, I use VAS6150 into the engine system, enter the output diagnosis mode, to drive the two air pump work, and found that the two air pumps do work with the driving command of the VAS6150, indicating that the author's inference is still more correct. So where is the next line of thought? Such a simple car, a simple fault code, it can not be ruled out many times, it is really embarrassing. In order to find out the real cause of the problem, the author once again combed the working process of the two air system: when the cold car starts, the engine control unit controls the power supply of the two air pump, the two air pumps run, the output of the fresh air with certain pressure, the output pipeline for the mechanical valve, the mechanical valve has certain pressure. When the air flow is opened, the fresh air enters the exhaust valve to mix behind, and the exhaust gas burns again, thereby shortening the preheating time of three yuan catalyst and reducing the emission. After carding, the author suddenly found that there were some negligence during the inspection process, although all the hardware parts (including one or two air pumps, pipes, mechanical valves) were checked, but the purpose of the hardware was to drain the fresh air through the outlet of the mechanical valve to the rear of the exhaust valve, and the author just neglected the fresh air. It has been sent to the specified position. It is also very simple to check whether fresh air is entering the designated location. Dismantle the mechanical valve first, connect the import A of the mechanical valve and the output line of the two air pump, then connect the two air pump relay short, control the two air pump, and find out how to operate the two air pump, and the exit B of the mechanical valve is not a little air out, so the root cause of the problem is explained. The two air pump pump fresh air can not open the mechanical valve, causing the engine control unit to control the two air pump work, because the mechanical valve can not open, in fact, the air flow sensor signal is still only X, not Z, control unit think that the air flow beyond the internal curve range, from this memory The fault code flow is incorrect and the engine discharge lamp is lit. But the mechanical valve can not be opened, there may be two problems, one is the two air pump pump pressure is not enough, and because the two air pump is new accessories, the related pipeline before the detection is not damaged, so the possibility of the two air pump can be excluded. The second reason is only the mechanical valve, directly let the accessories order the mechanical valve, change and test the car for half a month, the failure no longer appear, indicating that the fault has been completely eliminated. Fault summary: this simple fault, let the author also take a lot of detours, here deeply ashamed. The main reason for my introspection is to blow the mechanical valve with the mouth, and find that the mechanical valve can be opened (the mechanical valve is normal if it can be blown out or exhausted, which is in theory, but in fact it is completely wrong. " Because there is diaphragm spring inside the mechanical valve, and the spring is compressed to open the pressure of the mechanical valve, it has been calculated when the manufacturer is out of the factory. This is matched with the pressure of the two air pump. In fact, after installation to the car, the outlet of the mechanical valve is connected with the exhaust B and the exhaust, and the exhaust also has certain pressure, which leads to the machinery. The resistance of the valve is increased. And I take it down with the mouth to blow, in fact, there is a lot of strength, has been far more than the two air pump when the output of the relative air pressure, directly led to the author's judgment error, resulting in rework.