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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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Screen flickers randomly Computer then restarts.

Hi,

I have been having an issue with my computer today. It starts up fine and seems to function normally. After a while the screen shifts or flickers or turns white randomly (or all of the above). After that the computer shuts down and restarts.

Any idea what the problem may be?

Thank you,

Colin C

Update (02/18/2019)

I'm aware of the graphics card issue and replacement program. I had this issue dealt with a few years ago and my computer has been working fine up until this point. I ran terminal as suggested by Jonah. One of the previous shutdown causes is -128. The shutdown cause list suggests that it's "probably GPU related". What do you guys suggest is the best course of action ? Thanks for all your help!

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Ouch! I’m suspecting your dedicated GPU board is going on you.

Give this a try reboot your system and press and hold the Option (⌥) key is the screen stable then?

You might also try running diagnostics as well

Reference: Mac startup key combinations

Update (02/20/2019)

We don’t have a direct GPU board guide but this will get you 90% there Installing iMac Intel 27" EMC 2429 Dual Drive Kit (HDD or SSD) follow it to Step 33, pop off the SATA & power cables for the HD & optical drive, as well as the boards power connector so you can fully remove the logic board, carefully ease it out.

Once the logic board is out flip it over to locate the GPU thermal sensor

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Remove the screw holding the heat sink to the frame

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Then remove the four screws on the board

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Carefully lift the heat sink off

Your system came with either a MXM-A or MXM-B board

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Remove the two last screws holding the board in and ease it out of the socket.

Here are your choices for replacement boards:

  • AMD Radeon HD 6770 512 MB - P/N 661-5967
  • AMD Radeon HD 6970 1 GB - P/N 661-5968
  • AMD Radeon HD 6970 2 GB - P/N 661-5969

You might be able to recover the board by heating the board up to fix the cold solder joints that develop. You’ll need to apply some flux onto the board and using a heat gun heat up the board so the solder re-flows but don’t go to far! As you don’t want the components to shift. Some people just cook the board, I’ve found that technique is not as dependable as doing it properly. Here’s a time-lapse of someone using the heat gun approach YouTube - GPU Repair

You’ll also need to clean off the old thermal paste & thermal pads, and will need to put fresh thermal pads on the RAM chips (don’t use thermal paste).

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Running a diagnostic test right now! Thanks!

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@greenbuoy - Was the system stable in staying at the Startup manager screen?

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Yes I tried reboot with option. I didn’t have any issues there .

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Then your GPU has failed ;-{

This test forces the Intel processors built-in graphics engine to be used. You could also have run Target Disk Mode as well. As Target Disk Mode has a jumping Icon on the screen I wanted a more static image for the test.

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The system was stable at the Start-up manager screen.

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Holy Smokes! My computer has come back from the dead. I cleaned up the GPU and chips of the old thermal paste, baked it in the oven at 400F for 10 mins. I applied Arctic MX-4 to the GPU and K5 Pro to the chips. It works and I’m stoked!

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It sounds like a graphics card issue. It was a pretty common issue with 27” iMacs in 2011 with an AMD Radeon HD 6970M, so that’s what I’d lean towards especially if that’s your GPU.

The instructions in this guide can show you exactly why your computer is shutting off and would help us troubleshoot further.

If it does end up being a GPU issue, it’ll just need to be replaced.

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Thank you! I can see shutdown code -128 appear twice. I'm typing from the computer right now and so far no problems today.

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@greenbuoy yeah! I tried posting this comment earlier but the spam filters were removing all my posts yesterday haha. That probably indicates a GPU issue, I'd agree with everyone else. The only other thing I can think of that it may be is a RAM issue, you could try running memtest and see if it finds any issues. If not, you'll want to get that replaced.

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I just finished an extended hardware test. It took about 1hr 40 mins. It came back “No Trouble Found”.

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@greenbuoy Yeah, the Apple Hardware Test is not super great and usually doesn't find a lot of issues (like with the GPU), but the extended test does at least test every block of memory. Since that rules out RAM issues, that definitely indicates a GPU issue to me.

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Would the GPU not be included in the extended hardware test? I see there is an error code "4YDC" that should appear if there is an issue with the video/graphics card

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If your serial number ends with one of these designations

DHJQ, DHJW, DL8Q, DNGH, DNJ9, or DMW8

DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, F610

Your machine was part of an extended GPU card replacement program, which has now expired

https://9to5mac.com/2013/08/16/apple-ope...

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Yes, I'm aware of this program. I had the GPU replaced a few years ago.

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