Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
On Board Diagnostic (OBD): Function
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, i.e. it only registers malfunctions which have an effect on the electric signals.
OBD enables malfunctions to be traced quickly and efficiently using the VAG1551 or VAG1552 Scan Tool (ST) or the VAS5051 Tester 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. See 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, see SELECTING FUNCTIONS .
DTCs which may be stored in DTC memory will be indicated on the scan tool display. The information displayed can then be used in conjunction with the DTC table, which enables you to trace the cause of the malfunction and carry out the necessary repair quickly and efficiently.
The OBD distinguishes between "static" and "sporadic" malfunctions. If a malfunction which has been stored 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 read 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, see VAG1551 SCAN TOOL (ST), CONNECTING AND ENTERING ADDRESS WORD .
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.