Examples of a communication circuit test

Normal circuit 

1. Ignition switch is turned ON (II).
2. The driver's MPCS unit transmits a door lock switch message.
3. The MICU, relay control module, gauge control module, keyless access control unit and OnStar control unit* (if applicable) receive the door lock switch message.

   *: '05-06 models

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 ON (II).
6. The driver's MPCS unit transmits a door lock switch message.
7. The MICU, relay control module, gauge control module, keyless access control unit and OnStar control unit* (if applicable) expect to receive the door lock switch message, but since there is a break in the communication circuits, it is not received.

   *: '05-06 models

8. Each ECU that expects to receive the door lock switch message from the driver's MPCS 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 MICU, the relay control module, the gauge control module, the keyless access control unit, or the OnStar control unit* (if applicable). Each of these units will set "loss of communication" error codes for the message(s) they did not receive. There maybe 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 driver's MPCS unit sends the door switch message and door lock switch message during the communication circuit test.

   *: '05-06 models