OffroadFun 408 Posted March 27 (edited) My aux battery just died and I'm doing a delete and leaving it for now. (pull the fuse and disconnect the aux battery). The jeep won't start. I used a external jump still didn't start. My question is does if affect emissions in any way when you do this? Will it now fail? Edited March 27 by OffroadFun Share this post Link to post Share on other sites
4x4tographer 3,642 Posted March 28 2 hours ago, OffroadFun said: My aux battery just died and I'm doing a delete and leaving it for now. (pull the fuse and disconnect the aux battery). The jeep won't start. I used an external jump still didn't start. My question is does if affect emissions in any way when you do this? Will it now fail? Was your Jeep starting up BEFORE you disconnected the Aux and pulled the fuse F42? If “no”, then the bad Aux battery killed your main. You’ll need to replace it. A bad aux battery eats main batteries for lunch since they’re connected in parallel at all times EXCEPT during an ESS event (electronic start stop). If “yes” it might be something else - but could still be the main battery was damaged by the aux going bad. On the 3.6L the aux bypass procedure is to - disconnect the negative terminal on the aux - pull fuse F42 - start the Jeep. It WILL fail to start the first time. - The computer disconnects the relay that connects the Aux and Main in parallel (they’ll now operate as independent batteries). by pulling F42 the Jeep can’t reconnect them via the relay. - Turn off the Jeep - Try starting the Jeep a 2nd time. It should fire up off of the main. NOTE: When the Aux is bypassed, You will need to be religious about turning off the ESS feature on your Jeep every time you start it. The main isn’t designed to handle the load an ESS event will place on it and it will quickly kill your main. A workaround is to use a Tazer or ESS killer to disable the feature. Some people plug a wireless dongle into the trailer wiring socket, since the Jeep disables ESS when something is being towed. If you followed these procedures and the Jeep won’t start, you’ll need to replace the main. Will bypassing the Aux prevent the Jeep from passing emissions? No. However, you will need to drive the Jeep a few times for the computer to clear the fact you had a dead battery. I don’t know the number of starts for the JL to clear that “warning”, but if you replace a battery and go straight to Emissions, the Jeep will fail. Same with a jump. They’ll ask you to leave if the Jeep can’t start on its own. 4 Share this post Link to post Share on other sites
Curly 1,317 Posted March 28 What Ryan said unless you have an E=torque jeep. That is quite different. 3 Share this post Link to post Share on other sites
4x4tographer 3,642 Posted March 28 1 hour ago, Curly said: What Ryan said unless you have an E=torque jeep. That is quite different. Totally! There a so many different engine/battery variations on the JL it’s a little tough to remember them all! Share this post Link to post Share on other sites
OffroadFun 408 Posted March 28 turns out my jeep has already had the aux deleted.. I did replace the battery seems fine now. I have a tazer it keeps ess off for me Share this post Link to post Share on other sites
Andrew651 5 Posted April 17 On 3/27/2026 at 5:17 PM, 4x4tographer said: Was your Jeep starting up BEFORE you disconnected the Aux and pulled the fuse F42? If “no”, then the bad Aux battery killed your main. You’ll need to replace it. A bad aux battery eats main batteries for lunch since they’re connected in parallel at all times EXCEPT during an ESS event (electronic start stop). If “yes” it might be something else - but could still be the main battery was damaged by the aux going bad. On the 3.6L the aux bypass procedure is to - disconnect the negative terminal on the aux - pull fuse F42 - start the Jeep. It WILL fail to start the first time. - The computer disconnects the relay that connects the Aux and Main in parallel (they’ll now operate as independent batteries). by pulling F42 the Jeep can’t reconnect them via the relay. - Turn off the Jeep - Try starting the Jeep a 2nd time. It should fire up off of the main. NOTE: When the Aux is bypassed, You will need to be religious about turning off the ESS feature on your Jeep every time you start it. The main isn’t designed to handle the load an ESS event will place on it and it will quickly kill your main. A workaround is to use a Tazer or ESS killer to disable the feature. Some people plug a wireless dongle into the trailer wiring socket, since the Jeep disables ESS when something is being towed. If you followed these procedures and the Jeep won’t start, you’ll need to replace the main. Will bypassing the Aux prevent the Jeep from passing emissions? No. However, you will need to drive the Jeep a few times for the computer to clear the fact you had a dead battery. I don’t know the number of starts for the JL to clear that “warning”, but if you replace a battery and go straight to Emissions, the Jeep will fail. Same with a jump. They’ll ask you to leave if the Jeep can’t start on its own. Hi Ryan, After fully bypassing the aux battery, is there any reason to keep it installed? Share this post Link to post Share on other sites
4x4tographer 3,642 Posted April 17 1 hour ago, Andrew651 said: Hi Ryan, After fully bypassing the aux battery, is there any reason to keep it installed? No, it’s dead weight. I was too lazy to remove mine, but I will next time I have my fender liner removed on that side. It’s much easier to remove through the wheel well than it is removing the fuse box and the PDC items/connectors below the fuse box just to get to the battery. 3 Share this post Link to post Share on other sites
