Description of On-Board Diagnostics

TECHNICAL DATA
^ On Board Diagnostics (OBD) offers a number of troubleshooting possibilities for checking the vehicle.
^ Most causes of driveability problems can be diagnosed by evaluating information supplied to the Engine Control Module (ECM).
^ Malfunctions that occur while driving and that are recognizable by the system are stored and kept in Diagnostic Trouble Code (DTC) memory.
^ Several of the On-Board Diagnostics (OBD) options can only be used with the VAG 1551 or VAG 1552 Scan Tool (ST). The HELP button on the device can be used to get help during the test sequence.
^ Generic Scan Tool (ST) access of some functions is possible, refer to scan tool manufacturer instructions for proper connection and use.

SYSTEM OVERVIEW
^ Only with the use of the VAG 1551 Scan Tool (ST) are all of the On-Board Diagnostics (OBD) capabilities accessible.
^ The "Rapid data transfer" operating mode of the VAG 1551 scan tool is described in On-Board Diagnostics With Scan Tool.
^ Check Diagnostic Trouble Code (DTC) Memory function 02 should be carried out with the engine running if possible.
^ Output Diagnostic Test Mode (DTM) function 03 can only be carried out with the engine not running and the ignition switched on.
^ ECM coding capability using the VAG 1551 Scan Tool (ST) "Code Control Module", function 07
^ Fully Adaptive Oxygen Sensor (O2S) control, eliminating the need for the CO Fuel Trim potentiometer on the Mass Air Flow (MAF) sensor
^ Diagnosis of the Secondary Air Injection (AIR) system (where applicable).

NOTE: Vehicles that are equipped with a Secondary Air Injection (AIR) system can be recognized by a black plastic hose that runs along the front of cylinder bank 1.

MALFUNCTION RECOGNITION
^ A malfunction that is present for a specified time is stored as a static malfunction. If after a specified time the condition no longer exists, it is defined as a sporadic malfunction and identified with an "SP" in the display. The evaluation repeats continuously. If a sporadic malfunction does not reoccur within the next 50 engine starts, it is automatically erased.
^ If malfunctions that affect the exhaust and emission control are present they are indicated by illuminating the Malfunction Indicator Lamp (MIL) in the instrument cluster.
^ Each time the ignition is switched on, the Malfunction Indicator Lamp (MIL) must light up and then go out after the engine starts.
^ If the Engine Control Module (ECM) is disconnected, the voltage supply is interrupted and Diagnostic Trouble Code (DTC) memory will be erased.