What could cause no communication with ecu after engine swap?

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! After an engine swap, losing communication with the ECU is like trying to chat with someone who’s hung up the phone. Since you’ve already checked grounds and replaced the ECU, the issue could be with wiring harness connections or the CAN bus system. One tiny wire out of place can cause communication breakdowns. Also, the parasitic battery drain hints at a short or module staying active when it shouldn't. Double-check wiring from the ECU, especially grounds and power feeds, and see if any control modules are drawing power when the truck is off. If you need any further advice or if you'd like help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554!