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

WARNING: This page is about a different car, the 2013 Honda Fit, 2012 Honda Fit, 2011 Honda Fit, 2010 Honda Fit, and 2009 Honda Fit. However, it is still accessible from the selected car via links, so may be relevant.
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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