Symptom: a Touran 1.8T car. The driver to reflect the right front fog lights sometimes bright sometimes not bright, there have been repair workers to the left front fog light bulbs for the right front, right front fog lights better. Left front fog lights have made the same problems.
    Troubleshooting: After the first car to the left front fog lamp was checked. Directly connected to the battery. The brightness is normal, the lamp back to the left front fog lamp holder and then open the front fog light switch, the light bulb does not shine. The voltage is not measured on the lamp holder. Tour fog lamp system circuit is not complicated.
    According to the line map. After the current fog lamp switch is connected, it gives the U519 (in-vehicle network control unit) a request for a working signal. J519 according to this request, the two front fog lamps were output two working current. According to the figure found T5aa / 1 socket. No voltage is measured. And then up to find the J519 on the plug D / 2 no voltage.
    At this time the right front fog lights are bright. The voltage can be measured on the J519's A / 4 plug. It is speculated that the front fog lamp switch request signal has been received by J519. The right front fog lights are lit is J519 into a working state of a sign. And the left front fog lamp can not receive the output current of J519, then J519 internal this section of the work can be regarded as the object of doubt. In order to find the basis for this speculation or circumstantial evidence, in the J519 D / 2 this plug on the introduction of No. 30 Fire Line, and in the left front fog lamp holder connected to a small light bulb. Small bulb is bright. In order to prevent this line has a virtual place, and then connected to a large light, but also bright. When we are full of confidence to the original that was removed from the left front fog light bulb connected to the time, but not see it bright. Measure it off with a multimeter. But got a 53Ω (cold) resistance, and this resistance is much higher than the 55w fog lamp O. 6Ω (cold) resistance.
    Of course, it is directly connected to the battery is not bright, the actual received after the battery does not light. When all the lines are restored, replace the problematic bulb with a new front fog light bulb (which is bright on the battery when the fog light switch is turned on). Right front fog lights are bright, left front fog lights or not bright. And then test its voltage or not. This time suddenly remembered to close the front fog lights switch. When the front fog light switch is turned off and then on, the front and rear fog lights are lit. This is a feature of the fog lights and other driving lights on the Touran car when the circuit is broken or shorted. J519 vehicle network control unit after a short cold (light bulb does not work), heat (light bulb work) detection, it will automatically cut off the light bulb where the circuit current supply. This is also in the circuit diagram or the car can not find fog lamp fuses and fog lights relay reasons.
    Fault Summary: In this troubleshooting process, we also accidentally found that in the front fog light supply power detection, if we use a small light bulb or try to see whether the voltage exists or to replace the left front fog lamp, J519 also Can not supply current to small bulbs. This means that when an abnormal load occurs in the circuit. J519 control unit will also refuse to power supply.
    Left front fog lamp failure, the combination of instrumentation in the middle of the screen will tell you the text of the fault. But also cut off the front left fog light power supply. When troubleshooting. Hot and cold testing did not find any abnormal situation. The "fault display text" of the meter display is automatically deleted and the power supply to the left front fog is restored.
    The failure of the vehicle is mainly in the front fog lamp, it has a volatile cold resistance (measured 4 cold resistance, its quality and performance will be further studied). Up to 95kΩ. They are expressed in the light load, and some are in the J519 allowable range, most of them are not in the J519 work range, so the light will sometimes appear sometimes bright and bright phenomenon. Coupled with the road car J519 vehicle network control unit intelligent control method, leading to more peculiar failure. After this troubleshooting can be seen, according to the original routine inspection method to analyze the vehicle failure, mistakes are not also in trouble. When dealing with problems with new parts with new technology. We have to understand the characteristics of its work, but also to carefully observe and find things contrary to common sense, and to explore their practical significance. Only take the initiative. In order to obtain a greater harvest.