The failure of communication on the CAN-bus (i.e. fault code entries relating to bus communication in the individual control units may be caused by the following:
- Breaks in line (open circuits) or short-circuits in the communication lines
- Interference voltages in the vehicle electrical system caused for instance by defective ignition coils or ground connections
- Failure of the communication modules in the individual control units
- Failure of the voltage supply of individual control units (a battery voltage decreasing gradually when the battery is almost discharged can also lead to fault code entries as not all control units switch off simultaneously due to the voltages supply being too low).
A 120 Ohm terminal resistor is installed in each of two control units of the CAN network between the communication lines CAN-H and CAN-L. A resistance of 60 Ohm (parallel connection) can therefore be measured between the two communication lines in the control unit network. The lines can be checked easily by measuring the resistance with an adapter at one of the control units. When disconnected the resistances of both control units can be measured directly. Control units without terminal resistors normally show a value from 10 kOhm to 50 kOhm. The terminal resistors are accommodated in the ASC/DSC control unit and, depending on the type of engine, either in the instrument cluster or in the engine management.
Failure of a control unit or communication module normally triggers a fault code entry in the other control units connected to the CAN-bus.
Under certain circumstances, failure of a communication module in the control unit may paralyze the entire CAN-bus. Faults in communication with all other control units are entered in all control units connected to the CAN-bus.