Chosen Solution
Initial Symptom: System would work perfect for several hours. System would then suddenly start to get screen flicker for anywhere between a few seconds to several minutes and then system would power off. System would not power on for approx 12-24 hours after which time symptoms described would repeat. Troubleshooting (symptoms as per above after each step):
- Replaced LVDS Cable
- Replaced Power Supply
- Cleaned & Replaced Thermal Paste
- Alternate Screen Tested
- Alternate RAM Tested An alternate main logic board was sourced and the CPU transferred to the new board and the new board installed. The symptoms changed from the above to no power or activity at all. Looking at the diagnostic LED’s the first comes on when power is attached, no other diagnostic LED’s come on. CPU was returned to original main logic board and after installing the original board again the symptoms remained no power or activity at all. An alternate CPU has been sourced and tested with each main logic board and the symptoms remain no power or activity at all. I have tested with 3 alternate power supply signal cables. I have checked each board and I am 99.99% confident there is no damage to the CPU socket or pins. I have tested the power switch and have confirmed continuity and have even tested an alternate switch. Eliminating the other obvious I have tested multiple power cables and different power points. Proper static protection has been used so again 99.99% certain this is not the issue. I have also followed the deep dive troubleshooting but get to Step 12 which recommends replacing PSU, which I have done with the same lack of results. I am now at a loss. I have basically replaced every component apart from the fan and am still faced with no power, no activity and only the first diagnostic LED lighting up. Any help greatly appreciated and my thanks to you all in advance.
I’m wondering if your issue is really within the display. Apple had to do a bit of magic to drive the display, If you look at the iFixit teardown step6 you can see what I mean iMac Intel 27" Retina 5K Display Teardown. Update (09/10/2019) At this point too much has been altered so it will be hard to diagnose things. It’s even possible a cable or connector on the logic board has gotten damaged with all of the change outs. At this point I think you need a second known working system to leverage as a test bed to validate the parts. One of the tools I find helpful is the extension cable set Apple offers their service techs. While you can’t order it from Apple directly you might find a friend at an authorized service center who can get you a set for the original iDP (eDP) 4K & 5K cables and the cardboard display stand (you can make one your self).
