Passat OBD alarm lamp Fault Example

Failure phenomenon: a 2008 annual Shanghai Volkswagen Passat sedan, equipped with BGC engine and manual transmission, mileage 53000 km. OBD car users reflect light traveling in the combination meter alarm.

Check Analysis: First, it is necessary to briefly introduce OBD. OBD diagnostic system for a vehicle emission control, if a failure occurs on the quality of the emissions impact of the vehicle, will be stored in the fault memory and fault alarm lamp lights OBD. If a failure due to misfire, may result in the presence of catalyst damages, the OBD fault alarm lamp flashes. Fault light to these two different forms to prompt the driver of the vehicle fails, the need for timely to the service station for repairs. OBD causes alarm lamp are many, needed to determine the fault diagnostic failure range.

The author next car, first test found that failure is indeed the same as the user, so the use of mass-specific fault diagnostic VAS5051B diagnosis engine control system, a fault code found: 16825mdash; mdash; EVAP (fuel evaporative) emission control system error. According to the fault code, the initial judge OBD warning light is caused by faulty fuel tank ventilation system, where the specific point of failure also need further examination, in this, it is necessary to look at the function of the fuel tank ventilation system and how it works.

is a fuel tank ventilation system to prevent fuel vapor in the fuel tank into the atmosphere is provided by the adsorption of activated carbon, the activated carbon in the fuel vapor storage canister. Carbon canister outlet by a vacuum hose is connected to an intake manifold of the engine, and a canister solenoid valve to the control line between the two opened and closed. When the canister solenoid valve is open, the intake manifold vacuum generated by the negative pressure in the canister will be adsorbed fuel vapor drawn into the cylinder combustion, which also restored the activity of the activated carbon.

Passat after starting the engine, the canister solenoid valve comes into operation, when the oxygen sensor is started (the engine is in the warm state), the engine control unit turns on or off a canister solenoid according to the load and the speed signal valve. When the turbocharger is operating, the pressure in the intake manifold increases, in order to prevent air-fuel mixture in the intake manifold into the charcoal canister, the canister solenoid valve between the intake manifold and a mountingA check valve, the check valve is mounted at a lower position of the intake manifold. After the work of the turbocharger, the intake port fuel vapor by the turbocharger with charge air into the engine combustor together.


has a canister solenoid valve two pins, wherein a pin is a power supply terminal of the solenoid valve, powered by a fuel pump relay, the other a pin connecting the engine control unit, an engine control unit which is controlled by the ground or close the solenoid valve to open. When the solenoid valve is open, since the fuel vapor into the engine combustion, the concentration of the mixture gas rises suddenly when the voltage of the oxygen sensor will generate a transition (FIG. 1). The engine control unit according to the voltage change of the oxygen sensor, it is determined fuel vapor flow into the engine, that is, determines whether the fuel tank ventilation system work properly. When the engine control unit controls the solenoid valve is open, oxygen sensor voltage does not change, the engine control unit considers fuel vapor vent system failure, and transmits an alarm signal to the instrument cluster via the CAN bus, OBD lamp is lit, fault code and stores 16,825.

understand the working principle of the fuel tank ventilation system, further narrowing the scope of failure. The author of the supply solenoid valve and the control line is measured, no abnormalities, then the author analyzes the failure itself, or by a solenoid valve associated piping due to faulty. In order to determine the point of failure, I first check whether the line for some reason (e.g., vehicle accident) causes deformation, found no local deformation; VAS5051B diagnostic test function by using the element, driving the electromagnetic valve, solenoid valve and touch, in this case, the solenoid valve working feel and hear ldquo; da da rdquo; sound; Remove the solenoid valve connected to the pipe 2, when the electromagnetic valve, the compressed air inlet end to the solenoid valves blowing, the outlet end of the solenoid valve found no air flows out. Identify the cause of the failure so far, since the canister solenoid valve stuck inside, so that in a normally closed state. When the engine control unit controls its opening, since no fuel vapor into the cylinder combustion, the oxygen sensor no significant change in voltage, then the engine control unit will record the fault code 16825, OBD warning lamp lit simultaneously.

Solution: Replace the canister solenoid valve and clear fault code, then test, the fault disappears. 10 days after the customer visit, informed that this failure does not occur again.

reviewed: maintenance personnel when troubleshooting car, grasp the principle of the maintenance system, the faster and more accurately find the point of failure.

(Caoshou Jun)

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