I have a 2005 duramax that i replaced the longblock. The old one was knocking at 300,000 miles. I reused all original wiring harness and ecm. Now here is the problem, the truck will not spin over with the key, and has no communication with the scan tool, i have checked all fuses, all wiring connections, and used a remote starter button to verify the starter is working, the security light IS NOT ON. The batteries were disconnected for about a week during engine swap.

 

You may have killed the ECM, I would check all your wiring connections going to the ECM