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. Turn the ignition switch ON (II).
2. Clear the DTC with the HDS.
3. Check for DTCs in the A/T SYSTEM with the HDS.
Is DTC U0121 indicated in the A/T SYSTEM?
YES - Go to step 4.
NO - Intermittent failure, the system is OK at this time.

4. Check for Temporary DTCs or DTCs in the PGM-FI SYSTEM with the HDS.
Is DTC U0155 indicated in the PGM-FI SYSTEM?
YES - Go to the DTC U0155 troubleshooting in the PGM-FI system.
NO - Go to step 5.

5. Check for DTCs in the ABS with the HDS.
Is any DTC indicated in the ABS?
YES - Go to the indicated DTC's troubleshooting in the ABS.
NO - Go to step 6.

6. Turn the ignition switch OFF.
7. Jump the SCS line with the HDS.
8. Disconnect PCM connector A (44P).
9. Disconnect the ABS modulator-control unit 25P connector.
10. Check for continuity between PCM connector terminal A36 and ABS modulator-control unit 25P connector terminal No. 1, and between PCM connector terminal A37 and ABS modulator-control unit 25P connector terminal No. 17.





Is there continuity?
YES - Go to step 11.
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.

11. Update the PCM if it does not have the latest software, or substitute a known good PCM.
12. Turn the ignition switch ON (II).
13. Check for DTCs the A/T SYSTEM with the HDS,
Is DTC U0121 indicated in the A/T SYSTEM?
YES - Check for poor connections or loose terminals at the PCM. If the PCM was updated, substitute a known-good PCM, then go to step 6. If the PCM was substituted, go to step 1.
NO - If the PCM was updated, troubleshooting is complete. If the PCM was substituted, replace the original PCM. If any other DTCs were indicated, go to the indicated DTC's troubleshooting.