Symptom: Macbook (2011) won’t boot. Blue screen with lines instead of black. Stuck in a fail to boot loop

Solution (tested High Sierra Update 6)

Boot into Single User Mode - CMD-S held down on boot. When you see the terminal, hit enter a couple of times to get the prompt. Then run:

1
2

sudo nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00
reboot

Boot into Recovery Mode - CMD-R held down on boot. Choose the right language, then under the Utilities menu, start a terminal and type:

1
2

csrutil disable
reboot

Back to Single User Mode - CMD-S held down on boot. When you see the terminal, hit enter a couple of times to get the prompt. Then run:

1
2
3
4
5
6
7
8
9

fsck -fy
mount -uw /
sudo mkdir /amd-backup-kexts/
sudo mv /System/Library/Extensions/AMD*.* /amd-backup-kexts/
sudo rm -rf /System/Library/Caches/com.apple.kext.caches/
sudo mkdir /System/Library/Caches/com.apple.kext.caches/
sudo touch /System/Library/Extensions/
sudo nvram fa4ce28d-b62f-4c99-9cc3-6815686e30f9:gpu-power-prefs=%01%00%00%00
reboot

Back into Recovery Mode - CMD-R held down on boot. Choose the right language, then under the Utilities menu, start a terminal and type:

1
2

csrutil enable
reboot

src: https://grafxflow.co.uk/blog/mac-os-x/macbook-pro-2011-defective-gpu-fix