DTC Troubleshooting

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 to ON (II).
3. Check that DTC U0121 recurs.

Is DTC U0121 indicated?

YES - Go to step 4.

NO - Intermittent failure, the system is OK at this time.

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 the indicated DTC in the ABS system.

NO - Go to step 5.

5. Turn the ignition switch to LOCK (0).
6. Jump the SCS line with the HDS.
7. Disconnect PCM connector E (31P).
8. Disconnect the ABS modulator-control unit 25P connector.
9. Check for continuity between PCM connector terminal E11 and ABS modulator-control unit 25P connector terminal No. 1, and between E24 and terminal No. 17.





Is there continuity?

YES - Go to step 10.

NO - Go to step 15.

10. Reconnect all connectors.
11. Update the A/T software in the PCM if it does not have the latest software, or substitute a known-good PCM.
12. Turn the ignition switch to ON (II).
13. Check for DTC(s) in the DTCs/Freeze Data in A/T Mode Menu with the HDS.

Is DTC U0121 indicated?

YES - If the PCM was updated, substitute a known-good PCM, then recheck. If the PCM was substituted, go to step 1.

NO - Go to step 14.

14. Monitor the OBD status for U0121 in the DTCs/Freeze Data in A/T Mode Menu for a pass/fail.

Does the HDS indicate PASSED?

YES - If the PCM was updated, troubleshooting is complete. If the PCM was substituted, replace the original PCM. If any other DTCs were indicated on step 13, go to the indicated DTC's troubleshooting.

NO - Check for poor connections and loose terminals at the ABS modulator-control unit and the PCM. If the PCM was updated, substitute a known-good PCM, then recheck. If the PCM was substituted, go to step 1. If the HDS indicates NOT COMPLETED, keep idling until a result comes on.

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

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

YES - Repair open in the wires between the ABS modulator-control unit connector and the 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.