I would guess the video card has failed. I'd recommend running Apple Hardware Test in Extended Mode 2-3 times in succession. If the system reports errors then you have confirmed it's hardware and will need to take it in for repair or get the machine replaced. You can find the link to how to run AHT at:
If the iMac was getting too hot and overheating, this may have led to the GPU failing, obviously.
Every so often, it is good to clear out all of the vents, slots and ports of dust, lint and dirt. These do get clogged up over time and allow the iMac to get hotter and hotter.
The GPU in the 27 inch models is a socketed, replaceable chip that needs no soldering skills to remove.
There are DIY videos to show you how to tear down and reassemble you iMac, however, the issue is going to be procuring the replacement GPU. Apple and Apple Stores will not sell you one direct if you are not an authorized Apple repair center or Tech.
If you an find an authorized Apple repair center that isn't Apple, you might be able to work out a way to buy this from the repair center if they can order it.
If you find an alternate authorized Apple repair center, see how much they get for replacing the GPU.
They may very well be cheaper than Apple's repair rates.
Not sure which model 27 inch iMac you have, but if it had the base 256 VRAM GPU, you can replace it with the same one or you can upgrade to the 512 VRAM GPU.
I Just went to the WeLoveMacs website.
They have a service parts section where you can pick the year and model of your iMac and you get a complete exploded view of all of the parts of the iMac that you can order, if they can get these parts.
The standard 256 VRAM GPU is labelled.
661-5314, ATI Radeon HD 4670 256MB
512 MB VRAM GPU is labelled.
661-5315, ATI Radeon HD 4850 512MB.
Let me know what these will costs if they can get these as I may try a DIY tear down, myself, in the future, and upgrade my GPU to the 512 VRAM version. I currently have the 256 VRAM GPU in my iMac, now.
My 3-year extended AppleCare just ran out as of this coming Fri.
I Did a search on the WeLoveMacs site for the GPU and pricing.
They are selling the parts for full pricing!
The 512 VRAM GPU is $300
The 256 VRAM GPU is $349
Now that you know the type of video card and its part number for your 2009 iMac, I would suggest doing a general Google search to see of these cards can be had for cheaper.
I did a quick Google search and already came up with a site that sells this for cheaper.
So search the web for the best price for these GPUs.
Best of luck to you!
I Found yet another site that has the 512 VRAM GPU even cheaper, plus this site has videos showing how to tear down your iMac.
I've just baked it in oven and it works
What have I done :
- heat the oven at 180 degrees (celcius) for 15 mn
- Put my HD 4850 for 6 mn (3 mn on one side, 3 mn on the other one)
- the HD was in an aluminium sheet, carefully lied on a plate
- I removed everything from the GPU (papers, rubbers, thermal paste)
Now everything is working back... hope it will last
Hello, I've got the exact same issue with HD 4860 512MB.
Prior to buy a new one, I'll give a shot baking it!
Please could you be more specific regarding what parts you baked? Once you removed the thermal paste did you put it back? After the baking did you apply pressure on any component (I've read a post about appling light pressure on specific parts after baking)? Thanks!
PS did it last?
Hi there sorry to make you wait.
I removed the thermal paste, the small black foam and other tapes that I found there and there, anything easy to remove. I baked only the square graphic card, all the rest cooling metallic part were removed.
I bought some thermal paste (artic silver MX2 or something on amazon) to put a new one back, which I think is the thing to do. You could use the old paste, squeeze it and reforming it, but I'd suggest to put a new one quickly : it might be the quality of the paste which killed the card, who knows...
Me I didnt apply any pressure no, and as for today (1,5 month later), everything is still fine, on BOTH baked graphic card for my both iMacs