Taken from StackExchange
Thanks to LangLangC
For temperature and other improvements see https://gist.github.com/cdleon/d16e7743e6f056fedbebc329333d79df
Shutdown, unplug everything except power and hold
leftShift + Ctrl + Option + Power I holded for 5 seconds, but holding just for an instant should work
Command + Option + p + rUntil you hear the startup chime two times.
if you are on high sierra 10.13.6+ you might need to use
Command + rinstead
Command + r + scsrutil disablenvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00nvram boot-args="-v"rebootCommand + sIt might look as if it hanged, but press enter and you should see the shell (root#)
/sbin/mount -uw /mkdir -p /System/Library/Extensions-offmv /System/Library/Extensions/AMDRadeonX3000.kext /System/Library/Extensions-off/touch /System/Library/Extensions/rebootIt will show a bunch of text in the screen (don't be alarmed, let it finish) and then it will restart again In second restart it will show text in the screen again and then it will show normal login screen Your computer now should work properly (dGPU off and iGPU on, shown as i in gfxCardStatus
If an update that contains changes to the AMD drivers is about to take place it is advisable to move back the AMDRadeonX3000.kext to its default location before the update process. Otherwise the updater writes at least another kext of a different version to its default location or at worst you end up with an undefined state of partially non-matching drivers.
After any system update the folder /System/Library/Extensions has to be checked for the offending kext. Its presence there will lead to e.g. a boot hang on Yosemite and Sierra, an overheating boot-loop in High Sierra.
Due to not being able to use Parsec (Metal support), I installed Ubuntu on my MacBook Pro 8,3 and it works beautifully. Highly recommend it! It's a dual boot, so if I want to go into macOS, I still can. I have since moved onto using Sunshine/Moonlight and haven't tested it in macOS as yet.
Of course, you will still need to be able to turn the MacBook on in order to do anything and disable the dGPU. Usually a quick blast of a heat gun over the dGPU will get you going again. Just please remove the motherboard and anything you can before using the heat gun on it, or you can expect some melted plastic! The one time I was lazy and didn't feel like removing the board from the case, and boy do I regret it!