Most JLs with stop-start carry a small auxiliary battery alongside the main battery. When it weakens, stop-start quits and you get warning messages. On JLs where it's been bypassed or disconnected improperly, it can also be tied to network codes like U0121. Mopar BBAUX101AB is the current replacement.
Fitment
JL Wranglers with engine stop-start, per Mopar dealer catalogs. BBAUX101AB supersedes BBAUX101AA and several BBAUA-series numbers. Aftermarket Group 400/AUX14 batteries such as Interstate MTX-AUX14-ES are listed for the 2018 JL. Always confirm the number on your old part or run your VIN at a Mopar parts counter before you order.
Common failure symptoms
- "Stop/Start unavailable" or service stop-start messages
- Stop-start stops working even after a full charge of the main battery
- Electrical gremlins after an aux battery bypass
- Aux battery older than the main battery and failing a load test
Cross-reference
| Brand | Part number | Notes | Source |
|---|---|---|---|
| Mopar | BBAUX101AB | Current. Supersedes BBAUX101AA, BBAUA180AA, BBAUA200AA, AC and AD. | source |
| Mopar (older) | 56029759AB | Older number, superseded. | source |
| Interstate | MTX-AUX14-ES | AUX14 battery listed for the 2018 JL 3.6. | source |
Rebuild, repair or replace?
Batteries are replaced, not rebuilt. Load-test it first, since a weak main battery can also disable stop-start. Some owners bypass the aux battery instead of replacing it. Our U0121 guide covers a 2019 Rubicon owner whose U0121 traced to pulling fuse F42 and disconnecting the aux battery negative. Other owners report doing the same without codes, so wiring quality matters. A correct replacement is the low-risk path.
FAQ
Do all JLs have an aux battery?
It's tied to stop-start equipment; check under the hood or by VIN.
Can a bad aux battery cause U0121?
An improper aux bypass has been linked to U0121 in owner reports. See our guide.
Aftermarket option?
Interstate lists the MTX-AUX14-ES for the 2018 JL.
Related guides
Sources
Part numbers and fitment were checked against the sources above on October 22, 2026. Mopar supersedes numbers over time. If a newer letter suffix shows up, it generally replaces the one listed here. Back to the parts lookup.