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PATS / Engine Immobilizer Issue - Any Tech's Around?

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Ok so I'm in the middle (trying to near the end) of my engine swap. 2020 GT drivetrain into a 2016 ecoboost. Engine, Trans, PCM, BCM, BCMB, some other modules, and all wiring from 2020 GT installed. What I had hoped was enough to avoid PATS issues. And that seems to be the case, but there is also apparently an Engine Immobilizer Lock system that's separate from PATS that's preventing the new drivetrain from starting. I troubleshooted in Forscan as much as I could but pretty limited for this type of issue, so took it to the local dealer for them to look with IDS and see if they could figure it out. The tech Bill is a great guy and welcomed the challenge. He seems to have a pretty solid knowledge of how the modules communicate and how configuration works, but this one is pushing the limits of his knowledge/experience. So anyone here know anything about these systems? Appreciate the help, as I'm in for a full chassis rewire if we can't get this figured out. I knew it was a possible outcome of the swap, but still hoping we can figure it out.

-RCM Fuel interrupt not active, engine is getting fuel.
-Flashing green light on start button, indicates engine immobilizer activated.
-Cam/Crank signals not syncing properly. Looks like engine immobilizer may be intercepting crank signal -and obscuring it from the rest of the computer to prevent starting.
-Pressing it clutch causes loss of communication with PCM (another immobilization strategy?)
-PATS key reprogramming not available via IDS or Forscan (very odd)

What's been done/reviewed:
-Verify fuel pressure (low pressure and high pressure both) when cranking engine over (via relay jumper)
-Verified crank position sensor and wiring circuit operation/integrity
-Flashed PCM with latest firmware and tune via IDS with VIN# (2020 GT)
-Verified BCM part # is correct for vehicle (2020 GT) VIN #
-Tried to reprogram BCM/CEI via IDS with full credentials and get "invalid" response from Ford website (as best I understand from talking with the tech).

My Thoughts Questions:
-Does anyone know anything about the Engine Immobilizer system (not PATS)? Appears to be a separate system from PATS. Not having any DTC's and not having the ability to program new keys makes it difficult to trouble shoot. The workshop manual is also quite light as far as I can tell about the workings of this system.
-FCIM (radio/climate control panel with start button): is this part of the PATS or Engine Immobilizer systems? This part was kept from the 2016 car. Asbuilt updated to the 2020 car.
-IPC (Gauge Cluster): Is this part of the PATS or Engine Immobilizer systems? Original cluster from the 2020 GT was digital and sold before I bought the drivetrain. Bought an "analog" cluster from a 2020 GT MT and changed asbuilt data to turn off as much stuff as I could (adaptive cruise, Magneride, etc) to reduce the number of systems the car would be looking for and verify operability.

Next Steps Considered:
New BCM: Try installing a new BCM and hope the a fresh PATS and CEI configuration restores the "handshake" between the PCM and BCM.
PATS Delete & BCM Bypass for Starting: PATS delete of the PCM is easy. But the BCM still controls some starting signals. Would love to figure out which ones and just bypass the BCM altogether. Perhaps I can figure out which starting signals the BCM controls, separate them out from the BCM and wire in a new starting circuit?

Anyone (maybe @Black Boss) been through these schematics and harnesses enough to point me in the right direction for figuring out the CANbus controls/protocols of the starting circuit so I can try and figure out if making a new starting circuit independent of the PATS/CEI is feasible without a full chassis rewire?

Sorry for the novel, thanks in advance.

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