iMac won't boot green screen

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27" Retina 5K iMac 3.2Ghz Quad 24GB RAM, 1TB HD. iPhone 11.
I have a 2011 27" iMac and for the past year it's been a bit slow so I've tended to just leave it on all the time as it takes ages to fully boot but isn't too bad once it's up and running.

Anyway, the other day I was out of the room when I heard it reboot and it rebooted several times over the course of 30 mins.

When I eventually entered the room I found the screen was completely green with the exception of the apple logo which was on a perfectly square grey piece of screen. After the progress bar gets to within 3-4mm of completing, the screen goes completely grey, hangs for a few minutes and the reboots and so it goes on and on.

I tried resetting the SMC - no difference.
I tried resetting NVRAM - this removed the green screen, now on boot the background was white but still the apple logo is on a grey square. Again when it's nearly 100% the screen goes grey, it reboots and the the screen is green again.

I cant seem to get it to boot into safe mode at all.

I was able to boot into single user mode and repaired the boot drive using Fsck. A few changes were made and when I rebooted the screen was now white, still with a grey background to the apple logo and again it rebooted when nearly at 100%

I can't get it to boot into recovery mode but if I try internet recovery mode, it again gets to around 98% and then goes to grey screen and has to be rebooted

Occasionally when rebooting it trying different boot modes, the screen won't be green but a light yellow.

I Can get it to boot into Apple Hardware Test but even doing a full scan the only thing that is revealed is a slow fan on the hard drive, everything else is fine.

When I boot into either Singke User Mode or Apple Hardware Test I then have 5 green bars running down the screen roughly evenly spaced across the whole screen. If I leave it long enough, these often change colour and turn to snow.

So I'm thinking either the graphics card has failed or the logic board but wouldn't either show up in a Hardware Test? At 7 years old it's probably not economical to repair even if I could gets parts for it but is there anything else I can try or does anyone think it could be something else?
 

chscag

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2017 27" iMac, 10.5" iPad Pro, iPhone 8, iPhone 11, iPhone 12 Mini, Numerous iPods, Monterey
So I'm thinking either the graphics card has failed or the logic board but wouldn't either show up in a Hardware Test? At 7 years old it's probably not economical to repair even if I could gets parts for it but is there anything else I can try or does anyone think it could be something else?

I suspect you're correct. There are ways to repair or replace the graphics card in your iMac but it requires a complete disassembly of the display and removal of the logic board. The graphics card is removable and can be repaired or replaced. Replacing it is expensive; usually $250 or more here in the US. Someone there in the UK may be able to repair it but it is easier to just replace it.

The US site www.ifixit.com has instructions on how to access the graphics card. If you can find someone there in the UK who can do that work and not charge you an outlandish price, it might be worth repairing.
 
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2011 27" iMac, 1TB(partitioned) SSD, 20GB, OS X 10.11.6 El Capitan
That's hard on the nerves as I also have and use a 2011 27" iMac.

Mine didn't qualify but I'm sure there was an Apple extended recall for some similar models for some graphics problem, but it has expired here, but maybe your UK warranties are better and maybe extended.

Maybe give your local Apple Customer Service a call and inquire???




- Patrick
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