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It worked! Initially the machine turned on but without any processing activity. Now it is working but the supplied BIOS file is password protected. Can you unlock it? Here is the original file in case you need it:
This has no password, but it is not possible to boot into MacOS. I have tested a few versions, but in all of them the mac takes a long time to load the bar and then shuts down. I tested with disconnected peripherals as well. Recovery mode doesn't work either. Apple diagnostic does not show any problems. PRAM NVRAM Reset had no effect.
Do you have another BIOS file? Or at least the password present in the first one? Or do you think that it is no longer possible a BIOS problem, but something else on the board?
The last LED does not turn on. It just spins the fan, with no activity on USB and no image on internal or external monitor. I tested again with the original BIOS just to make sure it would happen again, and yes, it does.
According to the customer, the mac was used only a few times, and was left idle for a long time before trying to turn it on recently and it no longer works. In an Apple authorized service center, the customer got a quote for a motherboard replacement, since Apple doesn't perform any kind of repair in my country (actually, I don't know if it is like this only in my country).
The BIOS would be my only attempt on this mac, since there are no BRD files for this board and the board is in perfect visual condition.
"Transplant the ME Region from your backup to the new BIOS".
When I did this, the initial defect came back, so I was able to deduce that the problem was with the original ME region. After some searching I was able to find a ME region version that worked perfectly with the BIOS and now the mac works!
Just one question, is there a problem with using a ME region version that is different in Hotfix and Build? (In my case, 188.8.131.5281 was the original, and I put the version 184.108.40.20610). When I tested other versions that my software said were compatible, I was not successful (the mac turned on but lost some functions like AirPort, SSD recognition, etc.). The version I mentioned was the only one that worked perfectly.
Thanks for the help, I will make the final BIOS file available for download, in case it can help someone in the future.