"The battery was low after sitting, about 12.1V and then 11.9V after turning the key. We used the reliable old Denali to jump first at the battery in the trunk. This resulted in a big voltage drop for some reason"

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How is your BST?

Your symptoms (some of them) sure sound like a partially functioned BST.

I would also clear the codes and (if BST good) unplug the battery and ground the cables together then reattach the battery (fully charged) and go from there.

Sent from my SM-A730F using Tapatalk

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course