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DTC U0121: F-CAN Malfunction (ECM/PCM-ABS Modulator-Control Unit)

NOTE: Before you troubleshoot, record all freeze data and any on-board snapshot, and review GENERAL TROUBLESHOOTING INFORMATION .
  1. Turn the ignition switch to ON (II).
  2. Check for Pending or Confirmed DTCs with the HDS.

    Are DTC U0029 and U0121 indicated at the same time? 

    YES:  Go to the troubleshooting for DTC U0029 (see DTC U0029: F-CAN MALFUNCTION (BUS-OFF (ECM/PCM))  ).

    NO:  Go to step  3.

  3. Clear the DTC with the HDS.
  4. Check for Pending or Confirmed DTCs with the HDS.

    Is DTC U0121 indicated? 

    YES:  Go to step  5.

    NO:  Intermittent failure, the system is OK at this time. Check for poor connections or loose terminals at the ABS modulator-control unit and the ECM/PCM.

  5. Turn the ignition switch to ON (II), and watch the ABS indicator and the brake system indicator.

    Do the indicator come on and go off? 

    YES:  Go to step  6.

    NO:  Go to symptom troubleshooting for ABS indicator and brake system indicator do not go off (see DTC 86-FF: F-CAN COMMUNICATION WITH ABS MALFUNCTION ).

  6. Turn the ignition switch to LOCK (0).
  7. Jump the SCS line with the HDS.
  8. Disconnect the ABS modulator-control unit 25P connector.
  9. Disconnect ECM/PCM connector A (49P).
  10. Check for continuity between ECM/PCM connector terminal A3 and ABS modulator-control unit 25P connector terminal No. 1.

    Is there continuity? 

    YES:  Go to step  11.

    NO:  Repair open in the wire between the ECM/PCM (A3) and the ABS modulator-control unit, then go to step  12.

    Fig 1: Checking Continuity Between ECM/PCM Connector Terminal A3 & ABS Modulator-Control Unit 25P Connector Terminal No. 1
    G05952935Courtesy of AMERICAN HONDA MOTOR CO., INC.
  11. Check for continuity between ECM/PCM connector terminal A4 and ABS modulator-control unit 25P connector terminal No. 17.

    Is there continuity? 

    YES:  Substitute a known-good ABS modulator-control unit (see ABS MODULATOR-CONTROL UNIT REMOVAL AND INSTALLATION ), then go to step  12 and recheck. If DTC U0121 is not indicated after substitution, replace the original ABS modulator-control unit (see ABS MODULATOR-CONTROL UNIT REMOVAL AND INSTALLATION ), then go to step  12.

    NO:  Repair open in the wire between the ECM/PCM (A4) and the ABS modulator-control unit, then go to step  12.

    Fig 2: Checking Continuity Between ECM/PCM Connector Terminal A4 & ABS Modulator-Control Unit 25P Connector Terminal No. 17
    G05952936Courtesy of AMERICAN HONDA MOTOR CO., INC.
  12. Reconnect all connectors.
  13. Turn the ignition switch to ON (II).
  14. Reset the ECM/PCM with the HDS.
  15. Do ECM/PCM IDLE LEARN PROCEDURE .
  16. Check for Pending or Confirmed DTCs with the HDS.

    Is DTC U0121 indicated? 

    YES:  Check for poor connections or loose terminals at the gauge control module, the ABS modulator-control unit, and the ECM/PCM, then go to step  1.

    NO:  Troubleshooting is complete. If any other Pending or Confirmed DTCs are indicated, go to the indicated DTC's troubleshooting.

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