The following components can be checked individually, and most of them can be checked using suitable diagnostic equipment, such as the VAG 1551, Vag 1552 (or equivalent). Beginning with model year 1996 all passenger vehicles sold in the USA are On Board Diagnostic version II (OBD II) compliant and utilize a standardized 16-pin Data Link Connector (DLC).
The On Board Diagnostic (OBD I and OBD II) system relies on the Engine Control Module (ECM) to receive input from the various sensors. The system can be very sensitive such that the Check Engine/Malfunction Indicator Light (MIL) can be activated by rough roads, a loose gas cap or an empty gas tank.
Using a suitable Data scan tool (DST) can save precious diagnostic time. Many of the input sensor electrical connections are very difficult to access and will require using compatible electrical connectors to obtain an accurate reading.
A suitable DST can check for stored Diagnostic Trouble Codes (DTCs). Depending on the capabilities of the scan tool, the engine management system can also be checked while the engine is running, in a live data mode, without the chance of damaging the component(s), wire connections or their insulation.
Another advantage of using a suitable a live data DST, is that the systems can be checked how they interact with one another, and checked during initial start-up, monitored during the warm up period and at normal operating temperatures.
Once a repair is completed, the DST can clear all of the stored DTC's and if activated, can reset the Check Engine or Malfunction Indicator (MIL) Light.
There are 3 methods of communicating with the diagnostic computer in OBD II equipped vehicles. The OBD II protocol used by the Passat and Audi A4 is the ISO 9141 CARB system.
The Diagnostic scan tool (DST) electrical connectors are located as follows:
1990-94 Passat models:
1995-97 Passat models:
1996-97 A4 models:
1998-00 A4 and Passat models:
Diagnostic Trouble Codes, also called DTCs, are used to provide the technician with a digital representation of a fault which has been detected by the ECM. When such faults are detected, they are stored in the ECM and can be retrieved and used to help diagnose the faulty area and determine if a wiring circuit, connectors, or component has failed.
DTCs are retrieved and cleared with the use of a proper scan tool. These actions are described in their respective sections below.