Testing of a "Two-wire bus system" — AUDI

You have been requested in the fault table to check the bus:

Check how many control units are connected to the CAN bus using the corresponding circuit diagram.
Check the control units' multi-pin connectors for correct seating..
Connect a V.A.G 1551/1552 or VAS 5051 Fault Reader and press the 0 button twice for the "Automatic test sequence" function.

Before checking the bus lines, check to ensure that there is not a fault in any of the control units connected to the bus.

The term "functional defect" signifies a fault which does not directly affect the bus system but instead in some way or other disrupts the functioning of the system. This may, for instance, take the form of a defective sensor. The result is that the sensor signal can no longer be processed for bus-system data transfer. Such a functional defect has a direct effect on the bus system. Communication with the other control units which need the sensor signal affected is disrupted.

Is their a functional defect?

Rectify this first.

Note the faults and clear the fault memories of all control units.
Press the 0 and 6 buttons for the "Terminate output" function and confirm the entry using the Q button.
Terminate output from the fault reader using the "Terminate output" function.
Rectify the functional defects using the fault table in the corresponding workshop manuals.

Once you have eliminated all functional defects:

If communication between the control units is still disrupted, you will have to check the bus lines.

When searching for faults in the bus lines, differentiate between two cases:

Two control units are communicating via a "two-line bus system"
Three or more control units are communicating via a "two-line bus system".

Switch the ignition off.
Disconnect both control units
Check wether there is a breakage in one of the bus lines.

=> Circuit diagrams, fault-detection - Electrical system and fitting locations.

Check wether there is a breakage in one of the bus lines.

=> Circuit diagrams, fault-detection - Electrical system and fitting locations

Check wether one of the bus lines is short-circuiting to positive or to mass

No faults in the bus lines:

Try changing the control unit which can be changed with the shortest time input or cost
If control-unit communication via the bus remains disrupted, change the second control unit.