I have a 24" Early 2009 iMac.

During "intensive" use (such as Photoshop, WebGL, etc.), the system starts to become very unstable, resulting in bizarre graphics corruption and other weird behaviour. Ultimately, the machine freezes or crashes.

Somehow I managed to take a screenshot just before it went down, and I was surprised that it captured the carnage. I've censored (blurred out) some names to protect the innocent, but all the other glitches are part of the corruption.

I ran a temperature monitor in the background during use, and although it increased (as expected), it didn't seem to show unusually high readings for any components.

Anyone have any idea what could be wrong? Should I try reinstalling OS X from scratch, or is this definitely a hardware issue?

enter image description here


  • 2.93 Ghz Intel Core 2 Duo
  • 4GB RAM
  • GeForce GT 120 256 MB
  • OS X 10.8 (upgraded from Leopard, then Snow Leopard)


Having said that the components' temperatures seemed well behaved, I have started to notice that the problem occurs more readily when my room is warmer. Wondering if it is a heat-related problem...


Reinstalled OS X from scratch, but problem persists. I think this indicates a hardware problem. Sadface. Will open it carefully, hoping I'll find a busted fan or something silly like that. Otherwise, will take it to the shop - not keen to mess about with hardware even though it's out of warranty as the machine is still worth a fair bit I'd say.

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    I am seeing graphical glitches with OS X 10.9 such as this. The glitch can be carried across two monitors run with different graphics cards. Thus I suspect this is a software problem. Typically happens under load or heavy IOSurface using apps like Safari and Chrome. Commented Jan 7, 2014 at 18:41
  • @GrahamMiln that's awesome news ... perhaps I oughta reinstall os x from scratch ...
    – aaaidan
    Commented Jan 9, 2014 at 7:13
  • Is your serial number ending by 0KM ?
    – llange
    Commented Jan 9, 2014 at 8:26
  • Typically hardware faults would show artifacts at the pixel level. In your case, it's the UI elements that are being destroyed, which is not necessarily indicative of a hardware issue. In any case, the best thing to rule out software is to reinstall OS X and then create a new user account. If the problem appears in the new account, you have most definitely ruled out software. Unfortunately, the nature of these questions makes it very difficult for the community to offer focused answers as hardware diagnosis is often outside the scope of we are capable of doing over the Internet.
    – user10355
    Commented Jan 9, 2014 at 9:26
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    If you reinstall from scratch, be certain not to restore your machine from Time Machine or another backup. You could simply reintroduce the problem that way. As painful as it is, to get a truly clean install you have to reinstall all of your apps from scratch as well, re-authorize plug-ins, etc. Commented Jan 13, 2014 at 12:39

10 Answers 10


It's quite likely that this issue is caused by hardware (a failing GPU, most likely) but there's a possibility that it's caused by software.

To figure out where the problem lies, boot your computer into a clean install (either Mac OSX/Linux/Windows on a bootable drive, or use your "install Mac OS" install disk/partition). If you experience any kind of problems while using a clean install of any OS, you know the problem is hardware.

You should run a backup as soon as possible, then replace your GPU (Guide) (or take it for a repair). Only attempt to repair it yourself if you completely understand the instructions, if you're unsure, a self-repair may cause more damage

If there are no problems while using a clean OS, the easiest way to fix the problem would be to reinstall Mac OSX; remember to backup your files first so you don't lose anything.

  • Thanks, awarding this answer the bounty because of correctly suggesting that hardware is the problem, and linking to an excellent guide, relevant to my machine model. Cheers!
    – aaaidan
    Commented Jan 13, 2014 at 21:00

Backup first! Copy all your essential files, and make a Time Machine backup if possible.

It can be about software or hardware, it's hard to say.

I would first recommend creating a new user account and try there.

Then, reset NVRAM and SMC and try if that help.

The last good solution is Safe Mode and cleaning cache with apps like CCleaner or CleanMyMac.

The bad solution is just installing Mountain Lion on the same partition without cleaning your old system. If that doesn't work, upgrade to Mavericks and try. If that still doesn't work, you might need to do a clean install.

[EDIT: You can also just install to an external drive.]

Please remember to backup your Mountain Lion installation app if possible, or you might need to find is somewhere else later.

If all those above doesn't work, you might have to send it in for repair.

EDIT: Try BootCamp and run a benchmarking tool which pushes your CPU and/or GPU to the limit. That will show if it's a hardware problem.

UPDATE: Check out this Apple Discussion Thread, there's apparently people having problem like yours.

There's also a guide to testing your memory, something you can do if you want to.

  • Thanks for the answer! When you say "new user account and try there", what do you mean? See if the issue persists with a new user account?
    – aaaidan
    Commented Jan 7, 2014 at 6:34
  • @aaaidan Correct.
    – Shane Hsu
    Commented Jan 7, 2014 at 6:36
  • Graphical Glitches are most likely a Hardware problem. Your video card is probably dying. Commented Jan 7, 2014 at 10:04
  • Well, we should try everything to rule out a software problem, but ya, it's most likely hardware. A replacement will be, very expensive as it's probably soldered to the logic board along with CPU.
    – Shane Hsu
    Commented Jan 7, 2014 at 11:45
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    But I have to see from the look of the included screenshot, it could be software.
    – Shane Hsu
    Commented Jan 7, 2014 at 11:46

Installing the system from scratch isn't the best case. What I would recommend is if you have a spare external drive. Give it a partition of 25 GB or so and install an OS on it. Do not transfer any files into the new OS on the external. Instead you can open several applications (Grapher.app) and try to stress out the GPU and CPU through an external system. If the issue reoccurs then you are looking at most likely a GPU/video card issue some rare cases MLB.

If the issues don't come up then you most likely have a software issue or corrupted HD. If you have the DVDs that came with your mac then inserting the disk that has the Apple Hardware Test (it is printed on the disc) and boot the computer holding D. This will boot the iMac into Apple Hardware Tests and you can perform an extended test sequence.

Worse case you try an external system or AHT and the computer freezes or corrupts the video. Any AASP or Apple Store has more diagnostics tools to help troubleshoot. Be warned/advised There is a limit to how long a unit is eligible for service and how long parts are available so if the iMac is worth a few hundred bucks take it in to get checked and fixed. Before going in though, please have a backup all the files you need. I have over 6 years experience as Apple Certified Mac Technician so if you need any further clarification please let me know. I am being brief but as accurate as possible to make sure you waste as little time as possible.

  • Graphics glitches continue on fresh install of OS X. Looks like a hardware issue. What's MLB?
    – aaaidan
    Commented Jan 13, 2014 at 4:12
  • With your model it would require a graphics card replacement first and then further testing. Graphics card plus install and testing should probably be less than $400 but I am not in your country/area
    – Andrew U.
    Commented Jan 13, 2014 at 14:07

Speculated diagnosis:

I have seen this happen in quite a few macs since the switch to Intel architecture. Like what some others have suggested, I believe this is 90% most likely the initial stages of hardware failure (your GPU), and is directly related to cooling issues. In my experience, no amount of fresh-installing or hard drive replacing is going to fix this.

It is interesting to note that this is occurring on your NVIDIA hardware, as I have usually only seen the ATI chips fail. The cause behind this could be a flawed/abnormal manufacturing process or could be due to improper heat sink seating or inadequate thermal paste.

Unfortunately, it looks like you are going to be due for a repair soon. Your options there are either to replace the entire logic board (as others have stated, the GPU is soldered on), or have it repaired or "reflowed", a process where the logic board is removed and heated so all the solder melts and re-flows in an attempt to fix any shoddy contacts.

Possible fixes:

There are a couple band-aid type solutions you can try that I have had luck with in the past. Firstly, download a fan control app such as SMCFanControl, and set your fans to run at 100% all the time. It makes the machine noisier, but helps keep your mac cooler and more often under that magic threshold of it going berserk (on my old MBP it was 160F).

The second solution is to take the machine apart and re-apply the thermal paste and re-seat the heatsinks. It can be difficult if you haven't done that type of thing before but it is rewarding when it all comes back together. Obviously if you can afford it, it's best to take it to a repair shop (apple-approved only please!) and have them do it. They should only charge you about an hours' worth of labor and maybe a dollar or so for the drop of thermal paste they use.


I have this problem with my very similar iMac, which is why I'm trying this. I can't offer good news, I'm fairly certain it is a hardware problem and the fix is to replace or repair the card. For me that didn't seem worth the effort, given what I do with it so I'm making it an ssh server and have purchased a souped up mini to be my GUI workstation.

I tried most of the workarounds suggested here. Booting in safe mode, problem still there. Running the fan at high speed, no change. Fresh OS install, still there.

The only thing that ever seemed to make any difference, and not enough to be any real difference was that if I pulled the power cord and let it sit for a while, the next time I booted the amount of time before the troubles seemed a bit longer.

Everything I've read suggests that the problem is with solder somewhere in the system. It was poorly applied in the first place and wear and tear of heating and cooling has made it flaky.

Do you get errors in your console logs about problems with GL?


Backup is needed obviously. First backup, then sending it to Apple is recommended but if you do not wish to, you can disassemble it yourself. Clean it, and most importantly, you need to take out the ram and put it back in, the same with the heat sink covering your processors. The problem should lie with overheat and unstable ram.


The problem is your GFX/GFC Graphics Card. For many 2009 iMac's or macbook's this problem has occured. If you want to replace the part yourself, check out this thread right here : http://www.pcworld.com/article/225823/how_to_upgrade_your_graphics_card.html

Also make sure you want to buy the correct Graphics Card : https://www.google.com/#q=Graphics+Card+Mac&safe=active&tbm=shop

  • Upgrading a graphics card in an iMac is a very specific process, and won't be informed by reading articles about PC graphics cards in general (your first link). Also, there are generally only a few exact models of gfx card that work with any given iMac model, so even searching for "mac graphics cards" is way too broad. Check out my question (and answer) about upgrading the card in the mac relating to this question: apple.stackexchange.com/questions/117685/…
    – aaaidan
    Commented Aug 9, 2014 at 21:49

It was happening also on my old MacBook Pro. I have solved upgrading my RAM from 4GB to 8GB, because I've noticed that it was happening more often when I'd opened a lot of applications, and my free RAM was near to zero.

(It can happen due to shared memory with GPU)


I have similar problem with my iMac. Took it to Apple Store and was advised to stop using Google Chrome as it is the cause.

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    Wow. They actually told you to stop using Chrome? That seems fishy to me, at best. I would get a second opinion!
    – aaaidan
    Commented Aug 9, 2014 at 21:44

It's your graphic cards. Try updating them. http://www.apple.com/mac/

OS X 10.9 is only seen with this glitch. Also, download Adobe Player 11 or 12 beta. There might be a option at http://www.adobe.com

Also, use Safari or install Firefox, they are known as the fastest.

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