With VAG 1551 Scan Tool

On Board Diagnostic (OBD), general information
This is intended as a practical aid for technicians in the dealership when working on the Audi A4 and dealing with symptoms and malfunctions reported by the customer.
If the malfunction is related to one of the systems with On Board Diagnostic (OBD) capability covered here, this should give you the information you need to repair it.
The authors who prepare the service literature at the factory have exactly the same aim as the personnel in the dealerships: to make sure that customers are satisfied and to restore their confidence in the product.
The production involves a lot of work. Our intention is to describe and explain the technical features and repair procedures in such a way that repairs can be completed quickly and correctly. This is not always easy, and good results are only possible if you read this manual carefully.
The control module of a system with OBD detects malfunctions during vehicle operation and stores them as Diagnostic Trouble Codes (DTCs) in a permanent memory. The DTCs remain intact in the memory if the power supply from the battery is interrupted.
The OBD function relates to the electrical and electronic parts of a system; in other words it only registers malfunctions which have an effect on the electrical signals.
OBD enables malfunctions to be traced quickly and efficiently using the VAG1551 or VAG1552 Scan Tool (ST) in the "Rapid data transfer" operating mode 1. To do this, the control module must communicate with the scan tool via the data link.
The Data Link Connector (DLC) is the interface for communication between the control module and the scan tool.
To initiate On Board Diagnostic, connect the scan tool to the DLC and establish the communication link with the control module Refer to VAG1551 Scan Tool (ST), connecting and entering address word.
When the data link has been established, you can then select the required troubleshooting functions provided. These functions are programmed in the form of fixed diagnostic routines, which are described in the relevant sections of this manual. After first checking the control module version, start the troubleshooting procedure with "Check DTC Memory" function 02. All possible OBD functions Refer to VAG1551 Scan Tool (ST), selecting functions.
Any DTCs which may be stored in DTC memory will be indicated on the display of the scan tool. The information displayed can then be used in conjunction with the DTC table, which enables you to trace the cause of the malfunction and perform the necessary repair quickly and efficiently.
The OBD distinguishes between "static" and "sporadic" malfunctions. If a malfunction which has been stored in the memory as "static" is no longer present when the ignition is switched on, it is converted to "sporadic."
Sporadic malfunctions are identified as such by "/SP" appearing in the display of the VAG1551 scan tool.
A sporadic malfunction is erased automatically if it no longer occurs after a certain number of driving cycles.
Static malfunctions remain stored in DTC memory until the memory is erased using the VAG1551 scan tool.
If the control module has to be replaced, first check the control module version. This appears in the display of the scan tool together with the coding when communication is established between the control module and the scan tool.