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Model A1419 / EMC 2806 / Late 2014 or Mid 2015. 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (ID iMac15,1); EMC 2834 late 2015 / 3.3 or 3.5 GHz Core i5 or 4.0 GHz Core i7 (iMac17,1) All with Retina 5K displays

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Why is my iMac display not working on boot, but works in target mode?

My Late 2015 27" iMac had an SSD failure and RAM errors (according to apple running diagnostics)

Initially the display worked, but upon arriving at the Apple Store the display stopped working (though active backlighting glow), except it works when put in target mode (and on a couple other occasions)

The 7 year old OWC RAM was removed, with Apple factory RAM replaced

Thanks!

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First Target Display Mode is not accessing the systems graphics (GPU logic) as the system who is accessing yours is the one who’s graphics which is in use. What you have proved is your CPU logic and the network you are using is working just fine.

If the displace has a slight grey glow (in a darkened room) you’ve proved the displays backlight is working.

So that leaves us with either a graphics logic issue or the display or it’s data connection. Was the display taken off at some point? This could be a simple issue where the logic board data cable connection worked loose or there is some damage within the cable or the boards connection.

To prove which, I would plug in an external display via the systems Thunderbolt2 ports. If that works while the internal is failing you’ve isolated the issue to the internal displays connection or it’s own logic is bad.

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Hi Dan,

Thank you so much for your thoughtful and knowledgable response - I really appreciate it! The other modes I have seen activity have been with one or more of the following boot options

Cmd-R, Cmd-Opt-R, Cmd-Opt-Shift-R

Is it safe to assume these also work around the system graphics/GPU?

That is very informative. Yes, there is a slight gray glow to the screen. The display has never been removed and is still factory attached.

I normally do use a second display with the iMac, via Thunderbolt to DVI adapter. Yesterday I had plugged in that display to see if it would be triggered in any mode, and it was not. However, that said, since there is an apparent SSD failure and the system is not booting, would we expect the display to be active at any point? I don't think I recall the second display usually kicking in until after the boot process.

Am I understanding correctly that you are saying the display panel also has some sort of dedicated logic embedded or attached (outside of the graphics card)?

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Any idea if there could be any central component (logic board, defunct graphics card, etc) that could be causing other components such as the SDD to appear through diagnostics as failed? I cannot understand how I would have so many components go bad simultaneously. I have had the power for this machine run through an older APC Smart-UPS 1000, so I am inclined to think these issues were not caused by a power event.

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@rossn - So to be clear you do get a displace response when using the startup commands. If that’s the case then you’ve nailed the issue down to a bad Dedicated GPU chip. Remember the Intel processor Also has a graphics engine within it. So when using the startup commands you are in fact accessing it not the AMD Radeon R9 M380/M390 GPU chip which is part of the main logic board.

Even UPSs fail sometimes! Often the ground line is not properly wired or just worn out within your homes wiring. So the UPS can’t discharge the lighting or surge to ground.

It’s also possible the surge event can through the Ethernet or other ports which are wired to unprotected devices via other outlets.

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Ross zal eeuwig dankbaar zijn.
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