2007 Nissan Pathfinder No Communication with ECM - Low CAN Line Fault Detected

I own a 2007 Nissan Pathfinder that starts and runs normally, but I am unable to establish communication with the Engine Control Module (ECM). Initially, the vehicle wouldn't start, so I replaced the ECM with a new one. For a short period, it started and allowed communication via a scan tool. However, currently, no scanner or code reader can communicate with the ECM. I have verified that power and ground connections are secure and functional. I also recently replaced the Integrated Power Distribution Module (IPDM) with an updated unit, but the issue persists. All other modules show normal communication, and I can command outputs on various subsystems. Using a friend's Digital Storage Oscilloscope (DSO), I observed that the Low CAN line signal is severely degraded—appearing as noisy, erratic data—and does not mirror the High CAN line waveform. This suggests a potential failure in the CAN bus communication system. Can anyone provide insight into what might be causing this issue? Is it likely a faulty ECM, wiring harness problem, or a deeper CAN bus fault?