Chosen Solution
My wife’s Late 2015 iMac had been working fine until 2 weeks ago when the display turned black while she was using it. I have tried all the normal troubleshooting steps (reboot in safe mode, clear NVRAM, clear CMOS (hold power button for 10 seconds with power cord unplugged), reboot in recovery mode, etc.). Apple Support walked me through all their steps, then suggested I take the computer in for repair. A few months ago, I replaced the OEM Fusion Drive (why did Apple think these were a good idea?) with an OWC SSD, so I felt relatively comfortable opening the computer up to take a look to determine if there was anything I could diagnose. On startup, all four diagnostic LEDs light up, and the display backlights appear to be working. Internet sluething suggested the issue was with the display or display cable, so I ordered and replaced the display cable from the motherboard to the display. Now when the iMac starts up, it has a black screen, all diagnostic LEDs light up, but if I wait around an hour the “Question Mark Folder” appears. Since I’ve not had a Mac with “missing” system files impact the display before, I reseated the SSD I’d added, but nothing has changed (black screen on startup, all green LEDs, and Question Mark Folder after an extended period of time has ellapsed). I’d appreciate any advice or guidance people might have. My remaining thoughts are to swap the SSD out with a drive I know is working, replace the display panel assembly, or both. Since replacing the display assembly would cost about half as much as a new Mac, I’m hesitant to do so. Am I wasting my time and effort to keep this computer running?
At this point you’ll need to setup an external bootable drive from a working Mac with the same OS version your iMac is using. Then using the Option (⌥) when you restart your system to gain access to the Startup Manager to then select it to boot up your system. This should allow you to then backup anything you need to and then using disk utility delete the PCI SSD drive. Reference: Mac startup key combinations
