What is the cause of the U0121 code on my 2007 Honda Civic 1.8L and how troubleshooting?

Question

What is the cause of the U0121 code on my 2007 Honda Civic 1.8L and how troubleshooting?

2 Answers

answered Sep 14, 2018 by anonymous

Best answer

Erase the code first and see if it comes back, if it does you will have to go through the troubleshooting below.

DTC Troubleshooting: U0121 (107-7)

DTC U0121:

Lost Communication with ABS Modulator-Control Unit

NOTE: Before you troubleshoot, record all freeze data and any on-board snapshot, and review General Troubleshooting Information.

1.	Clear the DTC with the HDS.

2.	Turn the ignition switch ON (II).

3.	Check that DTC U0121 recurs. Is DTC U0121 indicated? YES - Go to 4. NO - Intermittent failure, the system is OK at this time.n

4.	Check whether any DTC is indicated in the DTCs/Freeze Data in ABS Mode Menu with the HDS. Is any DTC indicated in the ABS? YES - Troubleshoot for indicated DTC in the ABS. NO - Go to 5.

5.	Turn the ignition switch OFF.

6.	Jump the SCS line with the HDS.

7.	Disconnect PCM connector A (44P).

8.	Disconnect the ABS modulator-control unit 25P connector.

9.

Check for continuity between PCM connector terminal A36 and ABS modulator-control unit 25P connector terminal No. 1, and between A37 and terminal No. 17.

Is there continuity?

YES - Update the PCM if it does not have the latest software, or substitute a known-good PCM, then recheck. If the symptom/indication goes away with a known-good PCM, replace the original PCM.
NO - Go to 10.

10.

Check the communication line between the ABS modulator-control unit and gauge control module by the HDS.

Is there malfunction in the F-CAN line between the ABS modulator-control unit and gauge control module?

YES - Repair open in the wires between ABS modulator-control unit 25P connector and CAN communication lines.
NO - Substitute a known-good ABS modulator-control unit, then recheck. If the symptom/indication goes away with a known-good ABS modulator-control unit, replace the original ABS modulator-control unit.