Chosen Solution
Hello everyone, I have a bit of an issue. I’m experienced technician, but this was first 2017 5k 27 inch imac I was repairing at the component level. Problem was that there was no image output. Tried external usb-c, no chime, no logo, but fans work. I had previously repaired earlier 2015 imac with same fault, but found failed EFI , so I fixed it by replacing EFI chip. So I done it this time also. Bought a preprogrammed EFI chip for this particular logic board, soldered it on, tested connection - soldered great with solid connection. Powered on - and issue is still here. Then I noticed that only first two diagnostic lights light up. Now, does this mean that actual GPU might be fried? I know older imacs and macbooks had these issues, but new ones? I work in university as technician and they will not approve the entire logic board replacement, and quite frankly I was saving this iMac from getting binned. Any ideas? Suggestions?
It may, but it’s also a bit more complex than that. There are several systems that have to be in place to get this LED to light, even beyond the GPU being up and running. The GPU might be bunk, but it’s also possible something else is causing the issue. I’m not overly familiar with iMac boards, they are a bit different than MacBooks. But just a quick look at this schematic, there’s actually a lot going on electronically between LED 2 being on and LED 3. I don’t have access to schematics for this specific board (I was using the 2015 model for reference), but the first thing I would do is just give the board a once over, or twice over and see if anything looks weird, burned up, blown, etc. I would hazard to say that iMac boards are more likely to just have a simple short due to a blown cap, or a single component sort of failure, since they are MUCH less likely to have been liquid damaged. Or dropped.
I’ve found that sometimes, if the computer has a Fusion Drive setup, this issue can be resolved by removing the blade SSD. If you’ve tried everything else it might be worth a shot. Unfortunately doing so will break your Fusion Drive setup, meaning the remaining mechanical drive will need to be reformatted and macOS reinstalled.
