Faulty Communication

Every control module, apart from the central electronic module (CEM), has two diagnostic trouble code (DTC) types which relate to faulty communication (the central electronic module (CEM) has only one diagnostic trouble code (DTC) type).

These are: 

• Faulty messages
• Configuration error (not present on the central electronic module (CEM)).

Diagnostic trouble code (DTC) for faulty configuration is missing from the Central electronic module (CEM) since the control module is always the main control module in the network.

Faulty messages 

When a control module transmits faulty signals in its messages a number of times, the control module shuts down communication. This is referred to as the control module going to "Bus-off". A diagnostic trouble code (DTC) is stored in the control module that transmitted faulty messages, and the control module stops communicating on the CAN network. This is done so that the control module does not interfere with other communication on the CAN network. If the fault is permanent, it is not possible to communicate with the control module and there is no chance of reading diagnostic trouble codes (DTCs) from the control module that has shut down communication as long as the control module continues to be powered. If the control module which stored a diagnostic trouble code (DTC) is powered down (for example by switching off the ignition or if the fuse to the power feed is removed), the control module will attempt again to communicate when the control module is powered up again. When troubleshooting such faults, therefore, it is important to know what kind of power feed is used for the control module in question.

Configuration fault 

The central electronic module (CEM) transmits at regular intervals a control message to all control modules on the CAN network, This message includes, among other things, the ID number of the signal configuration currently being used. The reason for this is that all control modules on the CAN network must have the same signal configuration if they are to be able to communicate.

If the ID number of a control module's signal configuration does not tally with the ID number transmitted by the central electronic module (CEM), a diagnostic trouble code (DTC) is stored in the control module with the faulty signal configuration.

The control module which stores the diagnostic trouble code (DTC) will stop communicating on the CAN network but will still monitor traffic and respond to diagnostics communication.