VW Passat & Audi A4 1990-2000

On Board Diagnostics (OBD I/II)


In continuing efforts to improve air quality, the Environmental Protection Agency (EPA) amended the Clean Air Act in 1990. The Clean Air Act was originally mandated in 1970. The Clean Air Act has a direct impact on automobile manufactures whereby they are responsible to comply with the regulations set forth by the EPA. The 1990 amendment of the Clean Air Act set forth all of the changes currently being introduced on vehicles sold in the United States today.

In 1967, the State of California formed the California Air Resources Board (CARB) to develop and carryout air quality improvement programs for California's unique air pollution conditions. Through the years, CARB programs have evolved into what we now know as On Board Diagnostics (OBD) and the National Low Emission Vehicle Program.

The EPA has adopted many of the CARB programs as National programs and laws. One of these earlier programs was OBD I and the introduction of the Check Engine/Malfunction Indicator (MIL) Light.

The Bosch® engine management systems began meeting the OBD I standards and the check engine light requirements as early as the 1987 model year. This OBD I system allows diagnosis through the display of "flash codes" using the check engine light. The OBD I regulations were only the first step in an ongoing effort to monitor and reduce vehicle tailpipe emissions.

By model year 1989, all automotive manufactures selling passenger cars in the US had to assure that all individual components affecting the composition of vehicle exhaust emissions must be electrically monitored and that the driver informed whenever a related component failed.

Beginning with model year 1996, all vehicles sold in the US must comply with the OBD II requirements. OBD II standards require the monitoring of almost any component that could affect the emission performance of a vehicle and store the associated fault code (Diagnostic Trouble Code) and condition (type of failure such as signal implausible, component malfunction, or intermittent operation) in memory.

If a problem is detected and then re-detected during a later drive cycle more than once, the OBD II system illuminates the Check Engine Light in the instrument cluster to alert the driver that a malfunction has occurred. The flash code function of the Check Engine Light on OBD I vehicles has been replaced by the data acquisition capabilities of using a suitable Data scan tool (DST) and is not a diagnostic function of OBD II equipped vehicles.

The emission controls used on the 1990-00 Passat and 1996-00 A4 models operate in conjunction with the engine management system. When initially introduced, the mandated emission standards could be met by adding specialized components onto engines using the pre-existing fuel and ignition systems. These systems did help to reduce automotive emissions, however the engine's performance level and fuel efficiency were often compromised.

As the emission standards became much more stringent over the years, just adding emission control devices to existing engines and their fuel and ignition management systems would no longer suffice. The manufacturers were faced with the challenge of meeting these requirements by designing more efficient engines with improved fuel and ignition systems. These systems could not only meet or exceed the emission standards, the research and development of sophisticated combustion chamber designs and precise control of the fuel and ignition systems netted impressive performance gains and improved fuel economy.

The requirements of the mandated OBD emission standards included the ability of the system to be self-checking and provide a warning light to inform the driver if a component has failed that could adversely affect the operation and integrity of the system.

The emission control and engine management systems used on the 1990-00 Passat and 1996-00 A4 models can be broken down into 2 basic systems, On Board Diagnostic version 1 (OBD I) and On Board Diagnostic version 2 (OBD II).

The 1990-93 Passat models with the 2.0L 16V DOHC (9A) engine using the Bosch® KE-Jetronic, CIS-E (Continuous Injection System-Electronic) Motronic fuel management system are the only models covered in this guide with a fuel management system and ignition system that although controlled by the same Engine Control Unit (ECU), the 2 systems operate somewhat independently from one another.

With the introduction of the 1993 2.8L VR6 Passat GLX model and subsequent Passat and A4 models, the electronic emission controls, the fuel management system and the ignition system were combined into one engine management system that is controlled and monitored by one Engine Control Module (ECM). This combined engine management system manufactured by Bosch® meets the OBD I emission requirements and is referred to as the Bosch® Motronic Sequential Fuel Injection (SFI) system.

The OBD II emission standards became mandatory on all passenger cars sold in the US beginning with model year 1996. 2 of the requirements mandated by the OBD II standards included the installation of a standardized 16-pin Data Link Connector (DLC) and a post catalytic converter (downstream) Oxygen sensor(2) (O 2 S(2)).

The installation of a standardized 16-pin Data Link Connector (DLC) allows the connection of a suitable Data scan tool (DST). The post catalytic converter (downstream) O 2 S(2) sensor is used to monitor the operation of the pre-catalytic converter (upstream) O 2 S(1) sensor and the catalytic converter. The Bosch® Motronic system was upgraded to meet these standards.

The Engine Control Module (ECM) relies on input devices to monitor the engine's operating parameters. The ECM monitors the input signals and activates the emission devices, and controls the ignition and fuel systems to maximize the efficiency of the engine thereby reducing the emissions.

As mandated, both OBD I and OBD II equipped vehicles must be able to monitor it's own operation and warn the driver of an emission related failure. The Check Engine Light, also known as the Malfunction Indicator (MIL) Light, is illuminated if an emissions related problem is detected by the ECM.

Both the OBD I and OBD II ECM's have the ability to store fault codes referred to as Diagnostic Trouble Codes (DTC's). These codes can be read by using a suitable Data scan tool (DST), such as the VAG 1551 or 1552, or equivalent, or on OBD I systems, displayed via a blink code.

Accessing the DTC's can save valuable diagnostic time, although some systematic diagnostic troubleshooting is necessary. The systems can be checked dynamically and the component values read while the engine is running.

Using a suitable Data scan tool (DST) also allows the system to be checked without the chance of damaging the component, wire connections or the insulation. Many of the input sensor electrical connections are very difficult to access and will require using compatible electrical connectors in order to obtain an accurate reading. A scan tool that can monitor the system with the engine running allows the system to be checked during initial start-up, monitored during the warm up period and at normal operating temperature. Once a repair is completed, the DST can clear all of the stored DTC's and if activated, can reset the Check Engine/Malfunction Indicator (MIL) Light.

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). Only the OBD I DTC's can read using blink codes, however both OBD I and OBD II DTC's can be accessed by using a VAG 1551, VAG 1552, or suitable Diagnostic scan tool (DST).


As defined by the CARB 1968.1 regulations:

A Drive cycle consists of engine startup, vehicle operation and engine shutoff
A Trip is defined as vehicle operation (following an engine-off period) of duration and driving style so that all the emission related components and systems are monitored at least once by the diagnostic system except for catalyst efficiency or evaporative system monitoring which are monitored continuously.

A Drive cycle occurs any time the vehicle's engine is started, the vehicle is driven and the engine shut off. The vehicle may have or may not have reached normal operating temperature in a Drive cycle.

A Trip occurs when the vehicle's engine is started, the vehicle is driven long enough to achieve normal operating temperature and then the engine is shut off. As the vehicle reaches normal operating temperature, the engine management system goes into the closed loop mode. Closed loop means that the engine management system is adapting its settings based on the input received by the Oxygen (O 2 S) sensor. The other implication of closed loop operation is that the emission system has performed a system check on all monitored components.


The Federal Test Procedure (FTP) is a specific driving cycle that is utilized by the EPA to test emissions. As part of the procedure for a vehicle manufacturer to obtain emission certification for a particular model/engine family the manufacturer must demonstrate that the vehicle(s) can pass the FTP defined driving cycle 2 consecutive times while monitoring various components/systems.

Some of the components/systems must be monitored either once per driving cycle or continuously.

Systems and their components required to be monitored once within one driving cycle:

Oxygen (O 2 S) sensors
If equipped, Secondary Air Injection System
Catalyst Efficiency
Evaporative (EVAP) Vapor Recovery System

Due to the complexity involved in meeting the test criteria within the FTP defined driving cycle, all of the tests may not be completed within one customer driving cycle. The test can be successfully completed within the FTP defined criteria, however customer driving characteristics may differ and therefore the system may not always monitor all of the components and systems in one trip.

As mandated, the components and systems required to be monitored continuously include:

Cylinder Misfire Detection
The fuel system
The Oxygen sensors
All emissions related components and systems providing or receiving electrical signals to/from the ECM


The Check Engine/Malfunction Indicator (MIL) Light illumination is performed in accordance with the On Board Diagnostic (OBD) system regulations as per the CARB mail-out 1968.1 and must operate within the guidelines as outlined in the Federal Test Procedure (FTP).

As mandated by the CARB/OBD II regulations the Check Engine/Malfunction Indicator Light (MIL) is to be illuminated under the following conditions:

Upon the completion of the next consecutive driving cycle where the previously faulted system is monitored again and the emissions relevant fault is again present
Immediately if a catalyst damaging fault occurs (see Misfire Detection)

In accordance with the Federal Test Procedure (FTP), the Check Engine Light (MIL) is illuminated when:

A malfunction of a component occurs that could affect the emission performance of the vehicle and causes the emissions to exceed 1 1 / 2 times the standards required by the (FTP)
The manufacturer-defined specifications are exceeded
An implausible input sensor signal is present
Catalyst deterioration causes carbon monoxide emissions to exceed a limit equivalent to 1 1 / 2 times the standard (FTP)
A misfire fault occurs
A leak is detected in the (EVAP) evaporative system
The Oxygen (O 2 S) sensors observe no purge flow from the purge valve/EVAP system
The engine control module fails to enter the closed-loop operation within a specified time interval.
The Engine Control Module (ECM) or if equipped, the automatic Transmission Control Module (TCM) enters the limp mode (limp home) operating mode
The ignition key is in the ignition switch and turned to the ON position before cranking (Bulb Check Function) the engine