Error Management In the Controller Area Network (CAN)

Error management in the Controller area network (CAN)

General
The controller area network (CAN) is monitored by the Central electronic module (CEM). When the Central electronic module (CEM) detects a fault in the controller area network (CAN) a diagnostic trouble code (DTC) is stored in the Central electronic module (CEM). There are different types of diagnostic trouble code (DTC) depending on the type of fault.
Fault types which are processed are:
- No communication from the control module
- Faulty communication.

Note! To troubleshoot respective CAN-net, see troubleshooting for CAN-related diagnostic trouble codes for Central electronic module (CEM). For example, choose CEM-U000100/CEM-U000111/CEM-U000112/CEM-U000188 (HS CAN) or CEM-U001000/CEM-U001011/CEM-U001012/CEM-U001088 (MS/LS-CAN).

No communication from a control module
The Central electronic module (CEM) knows which control modules are in the controller area network (CAN) and checks that all control modules communicate. If a control module in the controller area network (CAN) does not communicate a diagnostic trouble code (DTC) is stored in the Central electronic module (CEM). There is a diagnostic trouble code (DTC) for every control module.

Faulty communication
The Central electronic module (CEM) continually monitors the traffic on the controller area network (CAN) when the network is in operation. If the Central electronic module (CEM) detects that the communication on the controller area network (CAN) is interrupted for any reason the control module can store a diagnostic trouble code (DTC). The Central electronic module (CEM) has a diagnostic trouble code (DTC) for every part of the controller area network (CAN) (one for HS-CAN and one for LS-CAN).
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
- Faulty configuration
Diagnostic trouble code (DTC) for faulty configuration is missing from the Central electronic module (CEM) when the control module is always the main control module in the network.
Faulty messages
The control modules continuously monitor the traffic on the controller area network (CAN). If a control module receives a message that it cannot interpret, it transmits an error message on the controller area network (CAN), a so called Error-Frame. The control modules also have a function to detect faulty messages that they transmit themselves. This stops interference with other communication on the controller area network (CAN). If there is significant interference on the network the control modules that are not communicating properly can shut themselves down (this state is called Bus-Off). When the control module closes itself down it cannot transmit or receive any information.
So that the vehicle does not suddenly stop and so that functions do not disappear entirely when controller area network (CAN) communication fails, certain control modules have a Limp-Home function. This means that control modules necessary for driving or safety (for example the engine control module (ECM) and transmission control module (TCM)) continue in a state of limited functionality with predefined or estimated values. As an example, if communication is lost with the transmission control module (TCM) the transmission will use a predefined gear. The intention here is that the customer can at least drive to a workshop. The control module remains in this mode until the power supply to the control module is switched off. When the power supply is connected the control module will make a new attempt to communicate.
Configuration fault
The central electronic module (CEM) transmits its configuration ID in the messages it transmits to other control modules. For the control modules on the controller area network (CAN) to communicate with each other they must have the same configuration ID. This is because a control module only listens for messages containing its own configuration ID. If the signal configuration of a control module does not correspond to the signal configuration of the central electronic module (CEM), a diagnostic trouble code (DTC) is stored in the control module with the faulty signal configuration.

Note! This means that a control module will store a diagnostic trouble code (DTC) if it does not "hear" the central electronic module (CEM) at all. This also applies if there is a fault in the control module software. If there is an open-circuit in controller area network (CAN) this means that a control module cannot "hear" the central electronic module (CEM).