Q: Red and green square dots?

I also have a Late 2011 21" iMac with the graphics card pixel anomalies. They started showing up right after my apple care expired in early 2014. After first finding this board (and posting as iMattPro), I was able to get apple to run diagnostics and they found a fault on the thermal sensor on the graphics card. They were unable to reproduce the issue in the store (because it is warm in the store and in the testing area). They cleaned out my computer. My problem didn't go away and as the winter temps dropped my issue worsened. So, I decided to give applecare another call. To my surprise my initial contact send me up the ladder and I was able to take it back into the Apple Store. I showed them video of my graphic anomalies and this discussion where people were baking their video cards. After only about 20 minutes they agreed after manager approval to replace my graphics card (quoted at $175) for free. As it was at the store for repair this afternoon the store called and tried to say it was a chrome issue and I needed to do a clean install! He was simply unaware that my fix had been authorized and was "trying to save me money". After I explained the situation, he replaced my graphics card with no clean install necessary. I have had my mac back all evening and no graphics issues or pixel anomalies.

So, there you go. Thanks discussion board! Do not bake your graphics cards. Just be really nice but very insistent and have apple replace your video card.

Apple extended coverage on the 2011's due to a documented flaw some graphics card designs.  You're not getting that with the 2010s.

They extended coverage for the 27 inch iMac only (because it was recalled) and that recall is now expired. I do not believe the late 21" iMac (at least from peaking over the genius' iPad) did not appear to have any such extensions for the graphics card.

It probably also doesn't hurt that this began right after your care expired and you had a history of them looking, mine waited until a full 2 years after Apple care expired. They had no interest in fixing it no matter how  nice I was.  Baking the card does work, and has been done with graphics cards outside of the Apple universe for quite some time.  It also can fix broken XBOX 360s with some red rings of death.  When all else fails you either have to spend money or bake your card.  Try baking first.

Just wanted to give one more thumbs up to the baking solution.  I had green and red squares appear randomly on my screen when the computer was cold.  I read the 33 pages of comments here thankfully before taking my way-out-of-warranty 2010 27' iMac to the apple store.  I pulled out the graphics card (ATI Radeon HD 5750 1024 MB), baked it at 385F for 7 minutes, applied new thermal compound on the head sink, popped it back in, went ahead and upgraded HDD to SSD while I was in there , put it back together, and it works beautifully.  Not a colored square in sight, cold or hot.  Thanks to those who suggested the idea.

As a side note, before I baked the card, I verified that the cold was the problem by downloading a program called NovaBench.  This program bench tests several aspects of the computer including the GPU.  Stressing these components warms them up, and indeed after starting up a cold mac (with colored squares) and running the test a few times, the artifacts stopped. Maybe that would be an acceptable interim solution for some. I just baked mine because I wanted to go in there for the SSD upgrade anyway.

The same problem, colored dots and squares, 2010, Radeon HD 5750 1Gb.

Problem first arrived today after installing 10.10.2 service pack, never seen before.

Crying...

Is there any "ready-to-use solution" for this problem?..

Exactly the same issue here on 2010, Radeon HD 5750 1GB system after updating to 10.10.2. Pink squares and graphical corruption. Thought perhaps graphics card was on its way out but seems too coincidental having read your post.

These threads are loaded with people who loaded mavericks, updates after mavericks, Yosemite, 10.10.2 etc....  It's most likely your video card.  It's just possible that it had time to cool while doing the update. 

Yesterday i "downgraded" my Yosemite 10.10.2 to Mavericks 10.9.6 with a USB-recovery, and the problem solved.

Now i have perfectly working Mac under any videocard load.

Jochta, try the same. I think Yosemite 10.10.2 is just underdone. 

Most likely it will be back.

Some day it will happen, i'm sure.

But not because of glitchy Yosemite 10.10.2.

And now reason for me is clear.

But you gave me a good advice - it's time to sell my Mac

I'm reading through this thread and I caught your location.  I'm at SDSU trying to diagnose the same problem for a librarian here on campus.  Frustrating!  I'll keep reading to get caught up with events here...

-Steve

I'm joining the list, just upgraded to Yosemite 10.10.2 and there are the red en green square dots

PS the funny trick by using your mouse to "click-select-drag" over the area showing the red en green square dots seems to get rid of them for over half an hour now...

For me the problem became worse than annoying colored boxes on my screen.  Eventually the system would freeze up completely. But it did start out as just annoying.

I had the same problem. First "click-select-drag", after some iterations - freeze 4ever.

===

Downgraded to Mavericks, and had the same problem (squares) twice, but disappeared after 'click-dragging', and no freezes.

Also i'm trying to understand the reason of the problem.

And thats what i noticed:

- I always have this problem on 'cold' iMac - 5 minutes after it starts;

- All the squares appear over background (not over program/browser windows);

- All the squares disappear after click-dragging, and do not appear when iMac is 'warm' (after 5 minutes);

- Any video stress-tests. benchmarks - the system goes slow, but NO squares.

- Never seen dots&squares on parallels Windows7 mode.

Is it just some kind of phantasmagoria ?