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Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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iMac won't boot. Logic Board? GPU?

I have a 27” 1312 iMac, with 16 GB of RAM with a replacement SSD for the original HDD.

All has been working fine until today. My system stays on 24/7, I went to use it and it had shut its self off.

Tried a re-boot, Apple logo appears, progress bar appears, gets halfway through and then solid grey screen, then the system reboots itself and will not stop unless turned off.

Cannot boot into safe mode or recovery mode. Reset PRAM and SMC. Tried external drive with OS, sees drive(s) and continues with external boot, but fails at same point (external drive boots MacBook no issues).

Apple support had no explanation.

Removed SSD and was able to boot my MacBook from the iMac SSD. Had iMac desktop so it’s not a drive issue. Installed new OS on a new SSD and installed to do tests.

Cannot load Apple diagnostics from iMac, but internet diagnostics loaded. Only issue was the HDD sensor as I’ve replaced with SSD.

Closest Mac store is hours away. Before I pack it up, just looking for suggestions. One store said it may be logic board. Another said GPU.

Been told that Apple can’t get parts as it is “Vintage”.

Is the machine salvageable? Thanks.

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Here’s one more test you can do to verify what your issue is. Reboot your system and hold the T key to get into Target Disk Mode. At that point your screen should have the FireWire or Thunderbolt Icon on your screen. If you get that then you’ve proved the issue is the dedicated GPU as the Intel graphic services are running. You’ll need a new GPU board.

Here’s the needed part:

Depending on your exact model you bought one of these three will work. Do keep in mind you need an Apple version of the MXM board as the firmware onboard is custom to Apple.

We don’t have a guide to replace the GPU board. This guide will get you mostly there Installing iMac Intel 27" EMC 2429 Dual Drive Kit (HDD or SSD) There are a few vids out there as well that can guide you.

Some people bake their GPU boards to try to recover them. This often works but I’ll warn you its not a fix only a band-aid and will likely fail in again. The root issue is the GPU board over heated and the tantalum caps on them tend to break down as well as the flip chip bonding within the GPU fails.

In any case you’ll need to clean and apply fresh thermal paste and thermal pads on the VRAM chips.

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9 Comments:

Dan...thank you for the reply. I did this and both logos appear on the screen, jumping around like a screensaver. So this is a GPU failure and not the logic board?

by

Yup! I had to take care of my dogs so I hadn't quite finished my answer ;-}

I've posted links to the GPU boards. Give me a few minutes to finish. Done!

by

Thank you. I have a Genius Bar appointment to confirm this issue. I appreciate your attention to this. Dogs rule!

by

Yes, that will do. What happened when you went to the Apple Store?

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