CAN-Bus System, Checking

The DTC table indicated you needed to check the bus:
- Using the relevant wiring diagram, determine how many control modules are connected to the CAN-bus.
- Check that the multiple connectors of the control modules are properly connected (unobstructed).
- Connect the VAG 1551 (or VAG 1552) scan tool and press the -O- button twice to select the "Automatic Check" function 00.
- Before checking the bus wires, make sure that none of the control modules connected have a function malfunction.

Note: In this context, a function malfunction means a malfunction that doesn't directly affect the bus system, but that interrupts the functioning of a system. This could be a malfunctioning sensor, for example. A malfunctioning sensor would mean that the sensor signal could no longer be used for data transfer in the bus system. This type of malfunction indirectly affects the bus system. Communication to the control modules that use the relevant sensor signal is interrupted.

Is there a function malfunction?
- Fix the malfunction before proceeding.
- Note DTC's and erase DTC memory in all control modules.
- Press buttons -0- and -6- to select "End Output" function 06. Press -Q- button to confirm input.
- End Scan Tool (ST) output using the "End Output" function.
- Refer to the appropriate DTC table in the relevant repair information for help in fixing the malfunction.
- You have fixed all malfunctions:
- Check bus wires if communication to the control modules is still malfunctioning.

While checking the bus wires, make a distinction between the two situations:
- Two control modules communicate via a "two-line bus system"
- Three or more control modules communicate via a "two-line bus system"

Two control modules communicate via a "two-line bus system".





- Switch ignition off.
- Disconnect both control modules.
- Check the bus wires for an open circuit.





- Check whether there is a short circuit between the bus wires.





- Check the bus wires for a short circuit to B+ or Ground (GND).
If no malfunctions could be found in the bus wires:
- As a test, replace one of the control modules. Select a control module that will take the least amount of time, and cost the least amount of money to replace.
- If communication between the control modules across the bus is still interrupted, replace the second control module.

Three or more control modules are communicating across a "two-line bus system".
- Analyze the DTC's stored in the memories of the control modules.

Example 1:
From the DTC's stored in the memories, you have determined that control module 1 is not communicating with control modules 2 and 3.











- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
- Replace control module 1 if no malfunctions can be found in the bus wires.

Example 2:








From the DTC's stored in the memories, you have determined that control module 2 is not communicating with control modules 1 and 3.
- Switch ignition off.
- Disconnect the control modules connected across the bus wires and check the bus wires for an open circuit.
- Replace control module 2 if no malfunctions can be found in the bus wires.

Example 3:





Using the DTC's stored in the memories, you have determined that none of the control modules are sending or receiving signals.
- Switch ignition off.





- Disconnect the control modules connected across the bus wires and check for a short circuit between the bus wires.





- Check the bus wires for short circuit to B+ or Ground (GND).
- If you cannot determine a cause for the DTC "Hardware malfunction", check whether the DTC is caused by one of the control modules.
- All control modules that use the CAN-bus are still disconnected. The ignition is switched off.
- Connect one of the control modules.
- Connect the VAG1551 Scant Tool (ST) or the VAG1552 mobile scan tool. Switch ignition on, and erase the DTC memory of the control module you just connected. End scan tool output using the "End Output" function 06.
- Switch ignition off and then on.
- Leave ignition on for 10 seconds. Then, read the DTC memory of the control module you just connected.
- If the DTC "Hardware malfunction" is displayed, replace the control module you just connected.
- If the DTC "Hardware malfunction" is not displayed, connect the next control module and repeat the procedure.