iMac 27" 2019 - Won't do (ventura) firmware upgrade after MB replacement

[overshoot]

Hello,

I have an iMac 27" 2019 for which I've ordered a replacement MB on ebay.
That MB came in with no serial number, stated it was an Apple replacement board.

I've swapped the CPU from my dead board to the new one, did a fresh install of macOS Mojave 10.14.6 on a 120GB SSD (Apple genuine NVMe drive from 2017).
From Mojave I did the minor software update, all went well.
Then I used the board serializer to flash my original serial number showing up on the stand, no issue so far.
Then I could see the Sonoma update from the "software update" menu and went for it.
Once the update starts, it will restart the machine a couple of times and ends up saying a firmware update can't be done.

I know the Macs are picky when it comes to firmware updates and you need genuine parts with no extra peripherals attached which I made sure.

Is there something I can check on the MB side to tell if it's caused by it?

 

[overshoot]

Actually my board model identifier is showing up as AAPJ1381,1 and that guy has the exact same issue I have:

Imac 27" 19,1 with replaces logic board - Apple Community

discussions.apple.com

 

[TwoH]

From what I’ve gathered when having firmware updating issues, the most common issue for it is installing onto a flash SSD. And it’s something I’ve noticed even with original Apple flash drives too.

Only way I’ve been able to make it update is to temporarily install a SATA cable and install another drive via SATA, in my case, a third-party SSD, for it to actually update the firmware.

If you only have a flash drive installed in your machine this could be the issue you’re facing

 

[overshoot]

It's an original Apple flash drive.
This machine was sold with a 2TB Apple SSD without any HDD.
I've tried with a standard HDD branded Apple and it didn't work either.

Since someone else had the same issue with an Apple replacement MB and has the same board identifier (which should iMac19,1) I assuming the issue lies here.

 

[overshoot]

Bios needed to be flashed. Found a clean ME version. Issue fixed.