Audi Cars 1999-05

General Information


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.

When performing repairs, Do NOT use any sealants that contain silicone to seal the intake area of an engine using Oxygen sensors. The silicone particles will not be consumed during combustion, thus the unburned particles will travel in the exhaust flow to the O 2 sensor. These particles can ultimately coat the sensor probe(s) and prevent or permanently damage the O 2 sensor operation. Additionally, Do NOT use electrical contact cleaner (or its equivalent) in the area of the HO 2 sensor harness electrical connector(s) because it could lead to corrosion damage of the sensor.

If available, the first step in performing component testing should be to check for any stored Diagnostic Trouble Codes (DTC's) using a VAG 1551, VAG 1552, or suitable Diagnostic scan tool (DST).

The Diagnostic scan tool (DST) electrical connectors are located as follows:

1990-94 Passat models:

Center console forward of the gear selector, under the gear selector cover or boot

1995-97 Passat models:

On the dash behind a trim cover between the radio and the steering column

1996-97 A4 models:

Center console in the rear ash tray, on the left, under trim a cover

1998-00 A4 and Passat models:

Behind a trim cover below the driver's side knee bar, left of the steering column

Click image to see an enlarged view

Fig. The Bosch® Motronic OBD I engine management system and related components. The OBD II system is similar

Click image to see an enlarged view

Fig. On Passat GLS and GLX models, the 16-pin Data Link Connector (DLC) is in the dash under a removable trim panel

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.