Chosen Solution

Managed to get my hands on a iMac 27” 5K (late 2015) as a bit of fun that is no longer used and the hard drive and blade SSD have been removed. I’m not sure if they’ve damaged anything else as the power button was sheered off, but I’ve fixed that. There is no sign of damage to the panel internal or external and all internal components look in good condition to the casual eye. My issue is I get no display. I can’t see a faint image on the screen and plugging in an external monitor via mDP>DP provides only “no input found”. There is no change in light on the screen in any way. The system does boot though;I get the Chime and it responds to keyboard inputs (I’ve reset the SMC and tried PRAM) The Diagnostic LEDs appear to work as expected:1 LED when power2 LEDs only if I try to boot with the LCD disconnected3 LEDs if the LCD connected As the original storage was removed I’ve been trying with and without any installed. I’ve not got a blade SSD to hand, but I’ve tried with both a spinning disk and SATA SSD.I’ve stripped the whole thing down, checking for obvious damage and replacing the thermal paste, but the only signs I can see is the lack of storage devices and brown staining around the wifi receivers. I’ve disconnected the 2x screen ribbon cables and reseated them. I’m not a mac specialist, but I’m assuming it should be able to present a image and ideally boot into recover with or without any SATA storage?I’ve been leaning to the issue being the panel itself, despite no sign of damage, but it is clearly working in some way if the system doesn’t boot (diag LED 3 + Chime) without it being connected. It’s also odd that there is no display on the external monitor which makes me think that it’s a GPU issue, but why LED 3 then?Any ideas before I return this and stop wasting my time? I’m not wanting to fork out the money for a new panel on the off chance.

Sadly your logic board has failed (GPU). 27” iMac Retina 5K Logic Board, 3.5GHz i5 QC, M295X 4GB, Apple P/N 661-0019227” iMac Retina 5K Logic Board, 4.0 GHz i7 QC, M290X 2GB, Apple P/N 661-0019327” iMac Retina 5K Logic Board, 4.0GHz i7 QC, M295X 4GB, Apple P/N 661-00194 You’ll need to replace it following this guide: iMac Intel 27" Retina 5K Display Logic Board Replacement