DTC Troubleshooting

DTC U1221 (35): BCM Module Lost Communication with DC-DC Converter

NOTE: Before you troubleshoot, record all freeze data, and review the general troubleshooting information.

1. Turn the ignition switch ON (II).
2. Monitor the OBD STATUS for DTC U1221 (35) in the DTCs MENU with the HDS.

Does the screen indicate FAILED?

YES - Go to step 3.

NO - If the screen indicates PASSED, intermittent failure, the system is OK at this time. Check for poor connections or loose terminals at the DC-DC converter and the BCM module. If the screen indicates NOT COMPLETED, go to step 1 and recheck.

3. Turn the ignition switch OFF.
4. Turn the battery module switch OFF.
5. Remove the IPU lid.
6. Check the DC-DC converter terminal (ORN wire) (A) connection.






7. Disconnect the DC-DC converter 4P connector.
8. Turn the ignition switch ON (II).
9. Measure voltage between DC-DC converter 4P connector terminal No. 2 and body ground.






Is there battery voltage?

YES - Go to step 10.

NO -
- Repair open in the wire between the DC-DC converter and the No. 3 ALTERNATOR (10 A) fuse in the under-dash fuse/relay box, then go to step 20.
- Replace the No. 3 ALTERNATOR (10 A) fuse, then go to step 20.

10. Turn the ignition switch OFF.
11. Disconnect BCM module connector A (40P).
12. Check for continuity between BCM module connector terminal A18 and body ground.






Is there continuity?

YES - Repair short to ground in the wire between the BCM module (A18) and the DC-DC converter, then go to step 20.

NO - Go to step 13.

13. Check for continuity between BCM module connector terminal A18 and DC-DC converter 4P connector terminal No. 4.






Is there continuity?

YES - Go to step 14.

NO - Repair open in the wire between the BCM module (A18) and the DC-DC converter, then go to step 20.

14. Substitute a known-good DC-DC converter.
15. Reconnect all connectors.
16. Reinstall the IPU lid, then turn the battery module switch ON.
17. Turn the ignition switch ON (II).
18. Clear the DTC with the HDS.
19. Check for Temporary DTCs or DTCs with the HDS.

Is DTC U1221 (35) indicated?

YES - Go to step 21.

NO - Replace the original DC-DC converter, then go to step 20.

20. Turn the ignition switch OFF.
21. Reconnect all connectors.
22. Reinstall the IPU lid, then turn the battery module switch ON.
23. Turn the ignition switch ON (II).
24. Clear the DTC with the HDS.
25. Check for Temporary DTCs or DTCs with the HDS.

Is DTC U1221(35) indicated?

YES - Check for poor connections or loose terminals at the DC-DC converter and the BCM module, then go to step 1.

NO - Go to step 26.

26. Monitor the OBD STATUS for DTC U1221 (35) in the DTCs MENU with the HDS.

Does the screen indicate PASSED?

YES - Troubleshooting is complete. If any other Temporary DTCs or DTCs were indicated in step 25, go to the indicated DTC's troubleshooting.

NO - If the screen indicates FAILED, check for poor connections or loose terminals at the DC-DC converter and the BCM module, then go to step 1. If the screen indicates NOT COMPLETED, keep the ignition switch ON (II) until a result comes on.

27. Reconnect all connectors.
28. Reinstall the IPU lid, then turn the battery module switch ON.
29. Update the BCM module if it does not have the latest software, or substitute a known-good BCM module.
30. Check for Temporary DTCs or DTCs with the HDS.

Is DTC U1221 (35) indicated?

YES - Check for poor connections or loose terminals at the DC-DC converter and the BCM module. If the BCM module was updated, substitute a known-good BCM module, then recheck. If the BCM module was substituted, go to step 1.

NO - Go to step 31.

31. Monitor the OBD STATUS for DTC U1221 (35) in the DTCs MENU with the HDS.

Does the screen indicate PASSED?

YES - If the BCM module was updated, troubleshooting is complete. If the BCM module was substituted, replace the original BCM module. If any other Temporary DTCs or DTCs were indicated in step 30, go to the indicated DTC's troubleshooting.

NO - If the screen indicates FAILED, check for poor connections or loose terminals at the DC-DC converter and the BCM module. If the BCM module was updated, substitute a known-good BCM module, then recheck. If the BCM module was substituted, go to step 1. If the screen indicates NOT COMPLETED, keep the ignition switch ON (II) until a result comes on.