Checking bus system if "No communication" fault occurs — AUDI

Notes:

If the connector is unplugged from a control unit connected to the bus system and the ignition is then switched on, a fault may be entered in the other control units connected to the bus system. The same applies if the power supply is interrupted, for example, by removing a fuse.
The "No communication" fault may be displayed by the operating and display unit -E87 for the connection between the dash panel insert and engine control unit.

Connect fault reader V.A.G 1551 (V.A.G 1552) => Workshop Manual, Page 01-7.
Switch on ignition.

Rapid data transfer
HELP
Enter address word XX
Indicated on display:

Press 0 key twice for "Automatic test sequence" function and confirm entry with Q key.
Use "Automatic test sequence" function to check all control units for "No communication" fault.

The fault is only displayed by the operating and display unit -E87 of the air conditioner:

Check for open circuit and resistance along data wires to -E87.

Resistance: max. 1.5 ohms

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

Notes:

Pay particular attention to possible loose contacts in wires and connectors.
If this fault is displayed simultaneously for the connection to the engine control unit and to the dash panel insert, the fault is located along the wiring to the -E87 or in the -E87.

If no fault can be found along the wiring, the fault is located in the -E87.

This fault is displayed in another control unit:

Interrogate fault memory of control unit with which -E87 cannot exchange data.
If different fault or additional faults are stored in control unit with which other control units cannot exchange data, eliminate these faults before checking bus wiring system.
Check part number together with associated index of control unit with which no communication is possible.

=> Parts List

Notes:

If there is a fault in a control unit, problems may be encountered when data is exchanged with other control units although the fault does not directly affect the bus system.
A number of control units are only configured for communication in the bus system from a certain part number onwards.
An open circuit in the power supply or in the earth connection to a control unit connected to the bus system may cause this fault to be stored.

Check for open circuits or resistance in the data wires and screening between the various control units connected to the bus system.

Resistance: max. 1.5 ohms

=> Current Flow Diagrams, Electrical Fault-finding and Fitting Locations binder

Check for open circuits and resistance in the power supply to the various control units connected to the bus system.

Notes:

Pay particular attention to possible loose contacts in wires and connectors.
This fault is only displayed for the control units which expect to receive a certain telegram from a different control unit and, consequently, cannot evaluate or do not detect this telegram.

The fault is also stored if the fault cannot be detected but is displayed at a different control unit.

The control unit with which the other control units cannot communicate should be replaced first.