Originally Posted by
Jon S.
Thank you for the additional information.
If all physical components are matched, and are in good working order, then the issues are likely wiring related.
Is the wiring harness new (from an aftermarket supplier), or did someone rework the used GM wiring harness ?
I have had several experiences where a corrupt wiring harness would cause nonsensical DTCs to be set.
Confirm the battery voltage circuits (constant 12 volts), ignition voltage circuits (key on 12 volts), and ground circuits. You may need to check every wire in the wiring harness for continuity, a short to ground (more common), or a short to voltage (less common).