On Board Diagnostic (OBD), Basic Principles

On Board Diagnostic (OBD) systems are operated by a control module which detects malfunctions and stores them as Diagnostic Trouble Codes (DTCs) in memory, which remains intact even if battery power supply is interrupted.

The OBD function relates to the electrical and electronic parts of a system; in other words it only registers malfunctions which affect electrical signals.

On Board Diagnostic (OBD) helps conduct specific troubleshooting. With the aid of VAS5051, VAG1 551 or VAG 1552 via operating mode 1 (Rapid data transfer) the functions of On Board Diagnostic (OBD) can be used. For this the scan tool and the affected control module must exchange data (communicate).

The diagnostic connector is the communication link between the control module and the scan tool.

If the communication link has been created, troubleshooting functions can be selected which are available for the system to be tested for troubleshooting.

Stored DTCs are displayed by the scan tool. Information displayed can be used with the DTC table for possible causes and necessary repairs.

On Board Diagnostic (OBD) differentiates 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 re-categorized as "sporadic."

Sporadic malfunctions are identified by "/SP" on the display of the scan tool.

If a sporadic malfunction no longer occurs after a specific number of driving cycles, it is automatically erased.

Static malfunctions remain stored in DTC memory until the memory is erased using the scan tool.

When replacing a control module, 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. Scan Tool VAG1551/1552 or VAS5051 Connecting/Selecting Function