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I've been using my iMac (mid-2011, OS X Yosemite 10.10.1) without any problems since I purhaced it. However, today a distracting yellow square pattern started to appear. It only appears over application windows, ie. I can't see any pattern over my desktop, but my browser is covered with it. It doesn't appear on every window, but almost all of them. The pattern moves if I scroll and it does change if I refresh a webpage. It also appears on screenshots I take.

I've tried rebooting and updating my OS without any change.

Example screenshot:

Example screenshot

What could cause this and is there anything I can do to fix it?

Update #1 All application windows (including OpenGL games) are either distorted or covered with yellow pattern. I said earlier that not all windows were affected, but I was wrong.

Update #2 Opening my computer using safe mode (pressing shift key right after the boot up sound) removes the yellow pattern, but introduces another screen bug. Now the windows are flashing. A white bar starts from the bottom of the window and moves to the top (at least 2-3 times per second).

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    Im afraid this is caused by a faulty graphics adapter, since the problem also shows up on the screenshot we can rule out the display and the cables. What you could try is switching to only integrated graphics, however I haven't found a way to do that on an iMac. – dennismuijs Dec 22 '14 at 14:43
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It seems to be a GPU problem.

Apple introduced Repair Extension program for some iMacs sold between May 2011 and October 2012. It is not on main Repair programs site though.

The affected iMacs contain serial numbers with the last four characters of DHJQ, DHJW, DL8Q, DNGH, DNJ9, DMW8, DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, or F610. Source

Check with this numbers and Apple to get a repair.

  • Thank you for your answer! My GPU is AMD Radeon HD 6750M (512 Mt). My serial number ends in "DHJF" which is not listed above. Looks like I won't be able to get a repair from Apple in this case. Is there a way to verify that it indeed is a GPU problem? – Elämäänsä Kyllästynyt Dec 22 '14 at 15:14
  • I say go ahead and try to get a repair under this problem anyway. There were mentions that not only this GPU that listed in a program are affected but other Radeons of that series too. So be polite but try to articulate that your problem is because of faulty GPU of the same type. Maybe try to call few times if needed or go to a Genius. – iskra Dec 22 '14 at 15:20
  • To make sure it is hardware and not software you have to use fresh install of OS X or Apple Service Diagnostic boot volume (for a portion of it you may just boot with Option + D pressed). Also try to clear NVRAM — support.apple.com/en-us/HT204063 – iskra Dec 22 '14 at 15:25
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As a follow-up to my earlier comment:
Have you tried resetting the smc and nv-ram?
If not, here is a guide on how to do it.
If it does not work, it might be the graphics card.
You need to find a way to switch to integrated graphics only, so you can verify.
There is no official way to do this but perhaps gfxCardStatus will work out for you.
Luckily there is an extended repair program for macs like yours just check the serials in iskra's answer

  • Thank you for your answer! I did try gfxCardStatus, but it refused to run saying "unsupported machine". I had no idea about smc/nv-ram! I will try to reset them and see what happens. – Elämäänsä Kyllästynyt Dec 22 '14 at 15:17

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