iMac bootup and display issue

Specs: Late 2012 Imac 27”

• Intel i5 2.9 GHz
• NVIDIA GeForce GTX 660M 512 MB

My colleague was having issues with this machine. Initially upon boot up, he was presented with the standard white screen, Apple logo, progress bar.

However, the progress bar would get to about 40% and would get stuck. Furthermore, the screen is covered in flickering pink/green lines which makes it difficult (but manageable enough) to read the screen. I have tried booting in safe mode, recovery mode, resetting NVRAM/PRAM, diagnostic mode, verbose boot, and booting to a separate USB with a fresh Catalina install.

Every key combination I’ve tried will result in the same white screen, Apple logo, and progress bar, which is able to load up to a different % before getting stuck (other than verbose boot, which I am unable to read any console output due to the screen issue). I decided to replace the SSD and inspect the internals for any visible damage. After removing the screen and replacing the SSD, I booted up, only to be met with the same issue.

I unplugged the Vertical sync and LED backlight cables again to ensure I had installed the SSD properly, and after booting up again, the screen won’t turn on (Completely black with no backlight either).

I inspected the vertical sync and backlight cables and saw no physical damage on them.

I also tried connecting to an external display using a Mini DisplayPort to HDMI adapter, which also did not work.

Furthermore, the bottom 3 onboard diagnostic LEDs still show that the computer is “Functioning properly” despite the screen not working.

If anyone has any experience with either of these issues (Boot Up issue or screen issue) or has any advice I would truly appreciate it