The DTC table may instruct you to test the bus:: Notes

By referring to the appropriate wiring diagram, check how many control modules are connected to the data bus.
Check that the multi-pin connectors on the control modules are properly seated.
Connect VAG 1551 scan tool or VAG 1552 scan tool and press button -0- twice to select the function "Automatic test sequence"
Before testing the bus wires, make sure that there are no malfunctions in any of the control modules which are connected to the bus.

NOTE: A malfunctionregistered in one of the control modules does not affect the data bus system directly, but can result from a malfunction of some kind in one of the systems. This may be a faulty sensor, for example. The result is that the sensor signal can no longer be processed for transmission via the data bus. A malfunction of this kind has an indirect effect on the data bus system, and will upset the communication with other control modules which require the signal from the affected sensor.