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Reinen
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99.9% of the time, yes.
Always replace both Main & Aux at the same time regardless if one of them still tests "good". At best it's nearly bad and will damage the new battery. Both batteries need to have the same age & use.
AndySpill
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While I completely concur, you may want to have each battery independently load tested first to see if one or more of the batteries is dying.
So we're clear, a load test is not about putting a multimeter up to the battery and testing voltage. It's a test that puts the battery under load strain to test that which a battery needs to do: deliver charge and accept new charge.
As @Reinen said, if such a test reveals either battery bad, you would be smart to replace both batteries most of the time.
Also so we're clear, the load tester cannot simply be placed on the terminals of the main battery. That would be a load test of both batteries as a pair in parallel and could yield good results when one battery is still bad, or detect problems, but not reveal which battery or both is the culprit.
Two black cables factory connect to the negative terminal of the main battery. One of them has as its other end the body ground on the front passenger's side under the hood. Leave that cable alone, keeping it connected to the negative terminal of the main battery.
If the other cable is disconnected, (whose other end BTW is the negative terminal of the Aux battery) then a load tester placed on the main battery's terminals WILL only test that battery. Make sure to keep that disconnected cable from metal parts under the hood.
If you move the negative lead of the load tester to that loose cable, yes, even as the positive side remains on the main battery's positive terminal, that will test the Aux battery (only).
Make sure to reconnect that loose cable after testing.
If both batteries pass a load test your ESS problem is not likely to lie with the batteries, although most of the time, that is in fact where the problem does lie. I'd just hate you to buy new batteries (plus installation if you don't do it yourself) and for it not to solve your problem.
58Willys
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Andy’s post is spot on. However, other things can cause this as well. Test batteries and check for trouble codes. I just had EES pop up, and it was a bad O2 sensor.
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OrneryBear
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That's not always going to help in this situation. Many people have taken their jeep to the dealer with battery issues and had poor results. they won't likely replace both batteries if one tests good. Also part of the problem stems from the cheap nature of the oem batteries. You're better off having the batteries properly load tested, and then going and buying quality non mopar battieries and installing them. Many swear by the quality of walmart batteries.
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