Examples of a communication circuit test

Normal circuit 

1. Ignition switch is turned to ON (II).
2. The door multiplex control unit transmits a door switch message.
3. The multiplex integrated control unit (MICU), relay control module, and gauge control module receive the door lock switch message.
4. The communication circuit test is passed.

   Since the door lock switch message was received by all the ECUs expecting to receive a message, the communication circuit between those units is OK. There are multiple messages sent and received by each ECU during this time to insure that the communication circuit is intact.

   Failed circuit 

5. Ignition switch is turned to ON (II).
6. The door multiplex control unit transmits a door switch message.
7. The multiplex integrated control unit (MICU), relay control module, and gauge control module expect to receive the door lock switch message, but since there is a break in the communication circuits, it is not received.
8. Each ECU that expects to receive the door lock switch message from the door multiplex control unit will transmit DTCs for the message that it did not receive.

Fig 1: Door Multiplex Control Unit Communication Diagram

Courtesy of AMERICAN HONDA MOTOR CO., INC.

Since there is a break in the communication circuit, the door lock switch message could not be received by the gauge control module, multiplex integrated control unit (MICU), or the relay control module. Each of these units will set "loss of communication" error codes for the message(s) they did not receive. There may be multiple communication DTCs if the unit that has become disconnected from the network would normally transmit multiple messages during the communication line test. For example, the door multiplex control unit sends the keyless panic message and door lock switch message during the communication circuit test.