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The 12V battery should be properly charged, then tested. You need a decent "smart charger" that specifically can handle AGM batteries....
Check the 12 V battery terminal connections are tight. You can't just twist it off and then shove it back on. You need to use a box...
No but what is the 12 volt say when nothing's been left on and the car won't start Do you have the CM monitor or the dash lights going...
Thanks, all. It needed a proper charge. It seems to be working now.