Auxiliary battery Issues

[wardd4]

Is it possible for a faulty auxiliary battery to cause a check engine light to come on and throw an o2 sensor code? I recently had spark plugs and the o2 sensors replaced but the start/stop system light came on followed by the check engine light and says it’s in “battery protection mode”. I’ve read the auxiliary battery can cause all sorts of issues.

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[Heimkehr]

A failing 12V battery may indeed cause a variety of seemingly unrelated electrical issues.

Resting voltage shouldn't be less than 12.4V; I personally like to see no less than 12.5V. Note also that resting voltage can test ok, and then fall off a cliff during a cranking event. Both load tests are required to properly determine the health status.

Now, when I did experience my single CEL episode -- P1128 -- it was a legitimate issue with a failed upstream O2 sensor. Replacing it was the necessary action.

What is the age of the batteries in your Jeep? They may be due for replacement. Replace both at the same time; otherwise, the ESS battery may eventually parasatize the main battery (as has been well-documented on this site.)