Mid 2011 iMac Graphics Problems with Yosemite

Thought i'd toss my 2 cents - i put up with the problem and kept getting worse, figured out running the machine hot mitigated the issue for a while, as long as the screen wasn't active, (when i went away, etc.) i could keep it running cool and as long as no one woke up the screen my remote applications could be accessed/files, etc.

Finally purchased a replacement and decided to go ahead (i've taken it apart many times for other upgrades) and tackled the baking of the graphics card just to see if it would work/replaced the, etc. thermal paste i had bought earlier to possibly do it and now.... the graphics card is fine... but my power supply was failing anyway. But that was interesting. Taking that thing apart isn't fun and only recommended for someone with steady hands and knows what they're doing. It took the entire afternoon. My wife said the iMac looked like it exploded from the inside out. I forgot to take a picture. So far 24 hours and graphics are fine, but i tore my hard drive's thermal sensor cable (i had replaced it with OWC's kit for an upgrade to 2TB so needed their external sensor kit)... and as always, ended up with an extra screw that i missed along the way putting back. LOL. So there will be a fan issue unless i use a software solution, but i'm going to recycle it anyway. Just was fun to try and do (since i am handy with taking computers apart).

Glad to be getting rid of the 2011, but it lasted me a long while - although could have lasted just a little longer - a testament to those Macs but i've had two defective macs (with defective video cards specifically) in a row and my wife's 27" had the 3TB drive recall and my older laptop had the screen/motherboard recall (i have to return it, they only did the screen and still has graphics issues. Graphics cards seem to be an achellies heel here on Apple's hardware?

Anyhow, was fun baking. Booted first try. Thermal paste is NOT FUN to deal with.

Already migrated to my new iMac 2017 - note: using thunderbolt adapter 2 to 3, setup internal network, you've got a super fast xfer method to transfer the system/accounts between old and new.

I know it's an old thread, but i saved it and finally got to do it! Cheers, John Q.

Apple ][ and forward, too many to list.

I know over 3 years have passed since your post re Java causing the random pixelated blocks, but when I recently installed High Sierra on my mid-2011 21.5 iMac (Radeon 6750 512mb) I started having the same problem. Sometimes total lockup requiring power off reset, but problem always coming back after a short time using the mac. After much grief, anguish, and telephone time with Apple Support, I found your post. Thank you. It solved my problem. Download of Java update directly from Oracle. No trouble for 3 days!

Run Apple's Hardware Test in extended mode 2-3 times back to back. If any errors appear that indicates the next step is a list to your local Apple Store or AASP for repair. You need to run the test 2-3 times back to back in extended mode because AHT is not the most reliable test and does not always pick up errors after the first pass. However if it detects them, they are correct.

Good luck.

I have exactly the same problem but with my 27" iMac mid 2011. After comes from sleep I get pixelated blocks in random areas that stay on background until open a new app window. Last night my computer frozen and got pixelated blocks all over the screen. After restart got the graphics problem and asked me to report it which I did. I restored Mavericks from my time machine and everything is working fine again. I'm gonna try a clean install of Yosemite but I'm gonna wait a few days before do that. Here is a pic of the pixelated block. I can't get this pic doing a screen capture.

I have the EXACT same problem on my iMac 27'' mid 2011. It eventually fails and I am unable to use the computer... This happened after Yosemite update. PLEASE HELP!

Last night I restored Mavericks in my iMac and everything is working fine again. I'm right now doing a clean install of Yosemite in my iMac. I will report tomorrow if the graphics issue is gone with the clean install.

Same issue here, only showed up after 10.10 GM update.

Did a Yosemite clean install and display problems persist. iMac was completely frozen this morning with artifacts on display. No third party applications installed, only Yosemite. Going back to Mavericks from Time Machine!! This really *****!! APPLE ARE YOU THERE?????????

enricoclaudio wrote:

Did a Yosemite clean install and display problems persist. iMac was completely frozen this morning with artifacts on display. No third party applications installed, only Yosemite. Going back to Mavericks from Time Machine!! This really *****!! APPLE ARE YOU THERE?????????

Apple does not read or assist in these forums, if you have feedback for them contact at www.apple.com/feedback.

I am having same problem with mid 2010 27" core i7 iMac with ATI Radeon HD 5750 with 1 GB of GDDR5 SDRAM model.

When I checked /System/Library/Extensions/ and compare with what's on maverick, I found a lot of extension is missing.

Yosemite may not needed these extension anymore but I saw other forum discussing about "missing display resolution choice on Yosemite" kind of thing.

such as like

http://apple.stackexchange.com/questions/151048/lost-ability-to-choose-3440x1440 -resolution-after-yosemite-upgrade

So I guess this could be related to Yosemite's wrong/buggy/or missing graphics driver and/or extensions.

I also noticed my secondly display no longer properly display (resolution wise). I used to have a lot more choice for scaled resolution but that is no longer available on Yosemite.

I just decided to stay with Mavericks until Apple release a fix for these display issues. I have Yosemite in my new 13" MacBook Pro Retina Mid 2014 and runs very well, unfortunately my Mid 2011 27" iMac doesn't like it...

Update: Partitioned my internal HDD and installed a fresh copy of the public version of 10.10, which I downloaded from the App Store. Setup my user and in my short usage time this morning, it showed no evidence of the graphics issue I experienced on my other internal drive. To me, this does indicate a software issue in the upgrade process I followed. Other users on here have reported this same issue going directly from 10.9.5 to the public release though, so not sure what that means for our models of iMacs.

Next step for me before Genius appt. would be to backup all of my data, wipe my primary SSD, and reinstall a fresh copy of 10.10 and migrate my data back. I will let you know what happens.

Interesting. I'm gonna give this a try. My Mavericks AMD6000Controller.kext is dated September 20, 2013 (10.9.5). I will report tomorrow is this fix my Yosemite installation in partition 2.

its going to turn out to be a software problem. I would guess that a fix will be rolling down the pike within a couple of days.