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Do you have a scan tool to check for codes set in the computer?
Any sub info codes on the 3000 code? Anything other then the P3000
Just to clarify you did get the orange high-voltage battery plug installed correctly? This is a three-step procedure. Unfortunately the P3000 code is a very general code. The car has been driven normally since the HV battery was replaced? In order to diagnose that code further you need the sub information three digit codes that would be accompanied by it. The 3000 code just means there's a general fault in the hybrid battery system. You need the sub information codes in order to determine what exactly is going on.
You have not done anything with the HV battery you only changed the 12V battery?
Remove the 2 battery cable ends from the 12v battery then isolate from the battery and place a jumper wire across the cable ends for 10 min. Then reinstall to the battery and see if the P3000 code is set. If not drive again and see if the code comes back. If it does there is a problem in the HV battery and I will need the Sub info codes set in the HV system to determine what is going on.