iMac 21.5" 2017 GPU/Display Issue After Bad Repair Attempt

Hello everyone,

I realize this is a very specific problem, and I may have made some significant mistakes during my recent repair attempt. However, I’m still eager to hear if anyone has any ideas on how to fix this without simply replacing the screen and logic board. Replacing these components would be prohibitively expensive, and at that price, I could likely purchase a working iMac instead. Additionally, I have a personal interest in reviving this machine, as I’d love to see it working again.

Here’s the situation: I recently bought a new SSD for my 21,5" 2017 iMac. After about 7 years of active use, it started overheating, and the CPU was constantly throttling, rendering the computer nearly useless—even with an external SSD I had set up due to the internal HDD slowing down drastically after just two years. Given the performance issues, I decided to open up the iMac, believing it was necessary.

Initially, everything seemed to go well, at least in theory, until I reached the reassembly phase. While reconnecting the screen to test the system, I accidentally yanked the cable right out of the eDP connector on the monitor. Although the connector was bent, it didn’t look too damaged at first glance. Unfortunately, I did rip one soldering pad off the PCB—specifically, the structural point on the connector that, fortunately, doesn’t carry any signal. I tried reattaching the structural point with super glue since it wasn't part of the signal pathway, carefully bent the eDP connector back into shape, and secured the cable using hot glue and tape. However, the display still showed no signs of life.

Checking the diagnostic LEDs, I noticed that the first LEDs were lighting up, but the next-to-last LED was dim, and the last one was completely off. This gave me some hope, after reading the documentation, that the GPU might still be functional.

The computer itself seems to be operational. I connected an external display and saw the question mark folder icon, which I took as a sign that the GPU was working. However, when I tried to boot into recovery mode, although I heard the boot chime, the system didn't fully boot up.

I also attempted to boot from an external Windows SSD. After a few tries and random key combinations, I managed to boot into Windows. But then, I encountered a new concern: in the Windows Device Manager, the Radeon Pro 555 GPU driver was stopped due to a problem (error 43?). This was extremely worrying, as I began to suspect that my attempt to forcefully reconnect the display might have damaged the GPU. The AMD driver also complained that the supposedly new GPU (which I obviously hadn’t installed) was not compatible with the drivers. I tried uninstalling the old drivers and reinstalling them from scratch, but after uninstalling the drivers and rebooting, I lost the video signal entirely after the Windows logo appeared. Reconnecting the display didn’t help, and there was no sign of life after the boot logo.

Another oddity is that the boot selection screen sometimes appears, but other times it doesn’t. This inconsistency adds to my confusion.

I checked the eDP connector fuse on the logic board, and it wasn’t blown, which gives me hope that my mishandling of the monitor connection might not be the cause of the GPU’s strange behavior.

I’m also considering whether the reset of the NVRAM could have triggered these driver issues, though I’m not sure. I don’t believe I could have damaged the GPU during the disassembly or reassembly process, as the only thing I did was clean off the old thermal paste with 99% isopropyl alcohol and reapply new paste.

Based on the diagnostic LEDs—and if I can trust the 2015 Apple repair documents for the 21.5" 2015 iMac and the 27" 2017 iMac—it seems that the GPU is communicating with the CPU and generating a signal, but it’s simply not detecting a display. If anyone has tips on how I can further check the health of the GPU or can confirm whether disconnecting the internal display (or connecting an external display) might cause such driver problems, I’d greatly appreciate your advice.

If it weren’t for this strange GPU behavior, my next step would have been to check the backlight fuse for the display, which is located on the back of the logic board. After that, I was planning to take the monitor to a repair shop to have the eDP connector properly replaced. I’ve already realized that attempting such repairs on my own, even seemingly straightforward ones, isn’t the best idea for me.

If I decide to take apart the iMac again to inspect the logic board, I could also check the GPU more thoroughly. However, I haven’t yet tried turning on the iMac with the screen connected but lifted up to monitor the diagnostic LEDs, as this is neither recommended by Apple nor seems like a safe approach. It feels like a recipe for causing even more damage.

Thanks in advance for any suggestions, diagnostics, or help. I really love this computer, and it would be a shame to lose it in such an unfortunate way, especially since I don’t currently have an alternative computer and can’t afford to buy a decent new one.