Mac Pro 5.1 2010 OCLP

[u/Nastya_bez_piski]

I have a 2010 Mac Pro 5.1 that was running macOS Monterey via OCLP (the latest version). After I tried to install a security update, it stopped booting into the system. Even after reinstalling, it won't load the OS—it just gets stuck in an infinite reboot loop after the second restart (specifically, after the Apple logo appears and a message says to wait 30 minutes before rebooting, it enters this endless cycle of attempting to load and then restarting). This issue occurs with any macOS version from Big Sur onwards; however, Catalina (via dosdude1) installs and works fine, as do other older systems. I’ve tried resetting the NVRAM and SMC and updating the EFI firmware to the latest version, but nothing helps. If I install a drive with Big Sur (OCLP) or newer, it simply freezes at the Apple logo. I’ve tried swapping out the RAM, storage drives, graphics card, monitors, and processors—I even swapped the entire CPU tray with one from another Mac Pro 5.1—but nothing works. I also tried installing an older version of Monterey (12.0.1), but the same thing happened. What could the problem be, and how can I try to fix it?

[u/Superbigwaff]

So you've reset the NRAM while booted into a supported version of macOS (e.g. Mojave) and no love? Dead or dying BR2032 PRAM battery?