Almost all modern Volkswagens use a sophisticated Bosch engine management system called Motronic. The Motronic system (also called the Digital Motor Electronics or DME) is the best overall fuel injection system that you can use when you consider price and performance. Ignition timing and fuel delivery are all controlled by a digital map stored within the main fuel injection (DME) computer. The computer takes input from a variety of sensors that are located on the engine: engine coolant temperature, crank angle, throttle position, exhaust gas oxygen (mixture), ambient air temperature and mass air flow. The DME is programmed from the factory with certain performance characteristics so that the engine will react well under a host of varying conditions.
As with any electronic device, components can fail, triggering problems with the system. The system is designed to react to these failures and indicate them to the driver, so that they can be fixed. If one of the computer's sensors is not working properly, then the computer may not be able to successfully identify the current state of the engine and choose the appropriate fuel mixture or timing advance level. When this happens, the fuel mileage drops, engine performance suffers, emissions increase and the car typically illuminates the 'check engine' light.
Pre-1995 VWs were equipped with what is known as OBD I (On Board Diagnostics Level I). Starting in '96, they were equipped with a more advanced version called OBD II, which was mandated by the US government, in order to standardize automotive repair and diagnostics. The OBD system is responsible for monitoring and checking all of the fuel injection sensors and systems in the vehicle and sending a visual error message to the instrument panel if it finds a problem or irregularity with one of them. If there is a problem with a sensor or component, the computer logs a Diagnostic Trouble Code (DTC) in the main computer until it is read and reset.
In order to accurately find the sensor and fix the problem, you will need to find out which error code is being triggered by the computer. There is no method to pull these codes out of your VW without the use of a scanner or a computer with diagnostic software. The factory VW software program is called VAG-Com. Using this program you can retrieve the DTCs from the computer and diagnose whatever problem you may have.
The VAG-Com software also allows you to dig deep into the various systems of the car and read values from the various systems of the car (airbag, ABS, engine, transmission, alarm, heating & A/C etc.).
You can also use a standard off-the-shelf OBD II reader available at any auto parts store, but it will only give you the DTCs without allowing any advanced features. Essentially, it points you in the general direction of the problem.
When you obtain the trouble codes, you can look them up in the factory diagnostic book or look up the code online. The factory book will have extensive sections describing the various faults and what is needed to fix them. Typically a web search will provide you with the information you need as well.
You'll need to access the OBD2 port under the dash in order to read the DTCs stored in the vehicle's computer. The port is located under the dashboard on the left side as shown here (green arrow).