The emission control system begins at the air intake and ends at the tailpipe. The emission control system includes various sub-systems such as the positive crankcase ventilation system, evaporative emission control system, the exhaust gas recalculation system and exhaust catalyst, as well as the electronic controls that govern the fuel and ignition system. These components are combined to control engine operation for maximum engine efficiency and minimal exhaust emissions.
Beginning in 1996 the U.S. government required a second generation diagnostic system, also know as On Board Diagnostic II (OBDII) on all vehicles sold in the United States. This Diagnostic Trouble Code (DTC) information is accessible to technicians without using the manufacturers scan tool. The manufacturers VAG 1551 VW scan can be used in generic mode to retrieve the trouble codes. The VAG 1551 is not the only code retrieval device that can be used. For information on other models, check with you local aftermarket parts supplier.
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.
Motronic Emission Control System
The CIS-E Motronic system used on the 16 valve engines is yet another version of the CIS-E fuel injection system. The system uses mechanical injectors, a fuel distributor, fuel pump and air flow sensor that are similar to those on earlier systems. Most of the electronic system controls are also the same. The major difference that the Electronic Control Unit (ECU) now controls the ignition system as well as fuel injection. The new ECU is equipped with an adaptive learning program which allows it to learn and remember the normal operating range of the mixture control output signal. This gives the system the capability to compensate for changes in altitude, slight vacuum leaks or other changes due to things such as engine wear. Cold engine drivability and emissions are improved. The new ECU also is capable of cold start enrichment without the use of a thermo-time switch.
The fuel injector pressure is higher for better fuel atomization and residual pressure. The threads on the new injectors are different so they cannot be interchanged with older units. Some other components such as sensors are similar to those used on the Digifant engine management system. Some of the testing procedures are the same but the parts are not necessarily interchangeable.