Q: Red and green square dots?

What seems to have been the problem for me (and how I solved this red dot/green dot issue) was to remove Google Chrome.

Like many of you I'm running a 27" iMac from mid-2010, Mavericks, w/12 GB RAM.

For what it's worth, two weeks after finally resorting the toaster oven, my screen issues are still gone.  Running Yosemite.  Default energy savings settings.  No more issues.  Good luck to all in finding a solution that works for you... after paying for something that should have just worked in the first place.  Ridiculous, Apple.  I expected much better.

I do believe their egos are getting in the way of their efficiency.

"Five minutes at 450. Good for pizza. Good for Radeon."

hahahahaha

iMac 11.1 i7 with 12 GB of RAM (mixed)  HD4850    So loooong out of warranty.

Problems started out with freezes in 10.6.8  Randomly. I was using some dodgy software so I always blamed it on bad drivers etc.

Now this is where it all started. I purchased Pixelmator from the App store and started to use it heavily. This caused images that I was editing to pixellate, and then spread to the Finder as welll and corrupting Preview (not able to save, not opening images etc.) and corrupting the Finder. (complete system crash)

There is a thread on the Pixelmator website forum (and them blaming Apple and their drivers BTW).

I deleted Pixelmator and everything was ok.  Out of curiosity I downloaded and installed Pixelmator again to see if it was really the culprit.

Bad move. Pink lines and pink blocks started to show up on my Mac and wouldn't go away. It wouldn't do a boot no matter what I tried. Eventually I managed to get into the AHT. (and it passed).. But after a restart pink blocks would show up again. I upgraded to Mavericks. Things did not improve.

And my computer would again freeze randomly needing a reset.

Upgraded to Yosemite.  (should fix things according to the Pixelmator team)

Used migration assistant to get my stuff back (which Yosemite failed to do correctly because my files are all over the place and only traceable with spotlight) But that's a different bug I guess ;-)

Did a restart again and pink blocks came back. (yes! I was already starting to miss them)

Seems that extreme use of my graphics card (Pixelmator team says that the app uses the graphics card in a different more extreme way and pushes it to the max) pushed it over the edge.

Tomorrow I will completely wipe my HD and install 10.6.8 and upgrade to Yosemite again. See what it does.

And what happens after a cold start in the morning because everybody speaks of no problems after long usage, but it's after a cooling down period when problems are starting to appear. (at least in my case)

will report back. (sigh)

Oh. Is this normal? only 3 Safari windows open and memory is 11,24 GB used?  While if you look at the actual memory per app it is not even a GB. Leak somewhere?

Oh. I like to add that I repaired disk permission in Yosemite and it came up with 2 display related permission repairs.

Thanks to Richlove and the others for the advice on baking the GPU. This is absolutely the fix for my mid 2011 27" iMac with an AMD Radeon 6770M. I have had the artifacts mentioned in the original post for some time now. I use this iMac full time for work, and as others have mentioned the issue was heat related. In the morning after waking the computer up I would have a ton of artifacts and as the heat increased they would disappear. I just dealt with the issue until my HDD was failing the SMART test so I knew I had to replace it soon. So I decided to do everything at once since I had to open it up. I replaced my HDD, added an SSD and baked my 6770M (390F for 7 minutes on top of aluminum foil balls). I then installed Yosemite 100% fresh. It's been about 5 days and I have had no artifacts. It feels like a brand new iMac. Baking is the way to go!

I'm glad this method worked for you, H4boxer. What I'm unclear about is why baking the old video card is better than replacing with a brand new video card... A good many have said they have had success with baking the card. I've also read on multiple occasions that the issue came back in a handful of those that have had their cards replaced by Apple. The ATI Radeon cards do seem to be aggravated by things like Transparency in Yosemite, and oddly cold environmental temperatures.

What is happening with the baking that's not present with a new gpu? Just curious... I'd rather not have to pull my iMac apart!

Had the same problem, here are three solutions that worked for me:

#1:

- download "Heaven" by Unigine (it's a free GPU benchmark program that will "heat up" the chip by making it work)

- run it every day, or as needed (green & purple squares will disappear for a few hours)

- sooner, or later get ready to either bake the GPU, or heat it up with a heatgun to melt the contacts (check YouTube for it)

#2:

- download "Macs Fan Control"

- tinker with the settings on the GPU, as to reduce, yet balance the heat evenly throughout the computer (squares have disappeared for two weeks and counting. Yay!)

- sooner, or later get ready to either bake it, or heat it up with a heatgun to melt the contacts (check YouTube for it)

#3:

- skip ahead to the baking right away.

It definitely has to do with the heat distribution on the GPU. There are tutorials on how to take your iMac apart from reputable sources.

Hope this helps.

PS: I think Yosemite & two built-in SSDs made my problem even worse. They require even less mechanical energy, which made my computer lose heat by efficiency. So far, regulating the heat with Macs Fan Control has been good to me.

I have a weird problem with my mid 2011 imac 21.5'. It has the 6750m 512mb, I have been through everything trying to solve the problem from replacing hdd and restoring back to mavericks and cleaning out the inside of my imac. My problem is not as bad as everyone else but it is similar. I only have some pixelations and square pixels when opening up a pixture in preview. I dont have any pixelation on a web browser or watching videos. I can even watch 4k videos without a problem. also if i have a web browser open and I open up a picture there seems to be more purple and green pixels. The pixels move with the window so its never on my desktop or anywhere else. It also shows up on the white work area when I open photoshop while a browser is open. My tempuratures seem to be normal so even though i can only guess its the video card its hard to seem why it doesnt show up on really tasking videos. I have never had a problem with freezing or shutting off and my desktop never has pixelation its only through photoshop and opening pictures. they seem to move around and disapear when i zoom in and out. Its very od but annoying. Please help. My last guess was maybe something with vram. also i forgot to mention that when i had yosemite 3 rows of 6 white dots that would flash on the top right corner of a youtube video but it would go away for good once i close the browser and open it up again. but it only showed up when I had it in full screen mode. Sorry for the very long message but If you have any ideas please let me know. I can also send pictures when I am home.

@gully

Well, from what I've experienced, I believe that the "normal" temperature for a defective GPU displaying all the issues above should be raised, i.e. "above than normal".

Running "Heaven" by Unigine did help for a few hours with the artifacts, until the GPU cooled down again.

All in all, given that I went through all the moves mentioned in this thread (reboots, App disinstalls, etc.), I have come to the conclusion that the green/purple squares are a hardware problem exclusively.

Go ahead and YouTube "baking imac GPU" and you will find this Mac repair guy who literally swerves a pocket torch a few inches over a 2007 iMac GPU chip to heat up the contacts. Once I saw that, I ventured into balancing my fans with Macs Fan Control in a way that kept my GPU toasty without melting the other components. I've not had one single green/purple square return since. No pop-ups, flickers, shutdowns, overheats, etc.

Disclaimer: I have a 2010 27" iMac with two SSDs, 32GB RAM and a ATI Radeon HD 5750 1024 MB sitting in a cold office. Your adjustments may very well vary.

Mid 2010 iMac 27" with ATI Radeon HD 5750. Red and green dots started to appear after upgrading to Yosemite. Mainly in the morning when colder occasionally severe enough to crash machine. Problem gradually got worse over a month. Each time after crashes Apple notice appeared inviting me to report graphics card, which I did. I wonder how many reports Apple now have? Having read all posts here, fiddled around with energy saving settings and other non-intrusive ideas. None worked. So followed advice of  Richlove and others and 48 hours ago baked my graphics card. So far no reappearance of artifacts. Will report back in about a week.     

hobblerman,

Is there a detailed walkthrough of how to do a bake?  I am a total idiot ... so would really like to see a video or pictorial walkthrough.

Or as a last resort some clear text instructions ...

The most I have done is add some crucial memory to my Mid-2011 iMac. 

Anyone know if a resources?  Did I miss something in our 31 page thread?

Thanks.

Hi Pound23

to to get the graphics card out I used iFixit instructions https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2309+or+2374+Graphics+Card+R eplacement/9553 then I followed guidance earlier in this thread about baking. Heat, time, thermal paste. Still working nicely after 4 days

Hello all,

I've enjoyed the 31 pages here (not!).

Anyway, thanks to all for the work. I'm not sure I'm ready to pull my graphics card and bake it out. (For what it's worth, my results are very similar to others here - cold office, no problems when warm, etc.)

Tom

<Edited by Host>