Macmini5,2 is there a way to downgrade Boot ROM?

[Andrii]

The story. After Security Update 2020-002 High Sierra my Mac mini mid 2011 with AMD HD 6630 immediately restarts on every go to sleep. Moreover, it restarts when idle, so now I need to run CPU-hungry background tasks. It's weird. I tried different OSes, different drives, PRAM and SMC, disassembling and cleaning from dust, replacing thermal grease etc. It works fine in safe mode. In normal mode it plays 1080p video from YouTube for 8 hours. But if I stop playing, it restarts in 5 minutes. It is not software problem, as I tried different OSes. It is not a hardware problem, because it works fine while CPU/GPU are high loaded. Problems appeared around a month ago, and after some research I found Security Update 2020-002 High Sierra updated Boot Rom to 135.0.0.0 at March, 20. Now I believe, new Boot ROM caused this weird thing. I found some other Macmini5,2 users have reported different problems after this update.

So I want to downgrade Boot ROM. I extracted MM51.scap file from SecUpd2019-002 and used this command to downgrade the Boot ROM:

sudo bless -mount / -firmware /Users/user/Desktop/MM51.scap --recovery --verbose

After restart Mac mini blinks with led, then quickly restarts few times, and then boots. But Boot ROM versions remain 135.0.0.0

What is wrong? Thanks.

 

[Andrii]

I have found that Macs from 2011 and newer do not let rewrite their Boot ROM. At least without external hardware SPI programming tool. So I can only wait for a new High Sierra Security Update. I hope it will contain a new firmware for my oldie.