Error Management In the Controller Area Network (CAN)

Error management in the Controller area network (CAN)

General
The control area network (CAN) is monitored and controlled 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 codes (DTCs) depending on the type of fault.
Fault types which are processed are:
- Electrical faults
- No communication from a 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 any of CEM-DF01 - DF17 depending on CAN-net.

Electrical faults
In the event of errors in the signal levels on the communication cables for the central electronic module (CEM) a diagnostic trouble code (DTC) is stored in the central electronic module (CEM). A check is carried out on each communication cable to the central electronic module (CEM), CAN L, CAN H on the low speed side and CAN L and CAN H on the high speed side. The faults that can be detected by the central electronic module (CEM) are:
- Short-circuit of controller area network (CAN) wiring to supply voltage
- Short-circuit of controller area network (CAN) wiring to ground
- Short-circuit between the control area network (CAN) cables
- Open-circuit in the CAN H.
This allows a total of 12 electrical diagnostic trouble codes (DTCs) in the controller area network (CAN).
The diagnostic trouble codes for electrical faults are CEM-DF0X for the low speed network and CEM-DF1X for high speed network.

No communication from a control module
The central electronic module (CEM) knows which control modules are in the control area network and checks that all the control modules communicate. If any control module on the control area network (CAN) does not communicate a diagnostic trouble code is stored in the central electronic module (CEM). There is a diagnostic trouble code (DTC) for each control module. The diagnostic trouble codes (DTCs) are CEM-DEXX.

Faulty communication
Each control module except for the central electronic module (CEM) has two diagnostic trouble code (DTC) types related to faulty communication. The central electronic module (CEM) only has one type of diagnostic trouble code (DTC).

These are:
- Faulty messages
- Faulty configuration.
There is no diagnostic trouble code (DTC) for faulty configuration in the central electronic module (CEM) because the CEM is always the Master in the network.

Faulty messages
The diagnostic trouble codes (DTCs) for faulty communication are XXX-E000 for the high speed network and XXX-E001 for the low speed network.
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). This is called an 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 a control module shuts itself down it neither receives or transmits 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 diagnostic trouble code for configuration fault is XXX-E003.
The central electronic module (CEM) transmits its configuration ID in the frames 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 diagnostic trouble code (DTC) XXX-E003 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).