iMac 2009 shows terrible graphic glitches!

What GPU does your iMac have in it?

You could post a picture or a link to some video of it, but I'd much sooner suspect a failing GPU rather than software with graphical glitches like random lines, blocks and/or freezing.

That does look a lot like a failing GPU. They can be replaced without replacing the whole logicboard in that model iMac. But they can be costly to replace.

The hardest part is opening and removing the logicboard. The GPU is on a video card and the whole card is replaced. You must use one from another iMac of the same or very similar model.

Yeah, that most definitely looks like a GPU on its death bed.

As Intell mentioned, replacement of the GPU itself is possible if you can find the part. Here is a guide from iFixit.

Some have also reported success by doing a reflow, basically putting the GPU in an oven at 200° for about 8 minutes to re-solder all the points on the board.

It's certainly worth a try before investing in a replacement part.

iFixit doesn't seem to have a guide for GPU replacement on the 21.5" but here is one for replacing the logic board on the 21.5 that should be enough to help you get the GPU out for a reflow.

Typically once a video card is damaged like that, it cannot be repaired or fixed.

garirry said:

I get it, but is it possible to prevent the graphic card from experiencing the issues by underclocking?

And a repeat of my other question: What usually causes the graphic cards to become corrupted?

Click to expand...

Sure it might, but afaik there is no way to underclock the GPU on the OSX side.

garirry said:

I get it, but is it possible to prevent the graphic card from experiencing the issues by underclocking?

And a repeat of my other question: What usually causes the graphic cards to become corrupted?

Click to expand...

In theory it sounds like a good idea.

Don't stress the GPU and it won't show sighs like that.

There is no way to underclock the GPU in OS X. However you can take some load off your graphic card by not doing anything graphic intense. (No Games, No Video editing, light flash work). It will die completely eventually.

garirry said:

The graphic card can't be under/overclocked because it's on OS X or is it because the graphic card itself is made to be incompatible? Can I just install Windows on Boot Camp and underclock the card?

Click to expand...

You can usually change the video card clocks on a Windows Bootcamp (I've overclocked my card that way). A utility like Sapphire Trixx or MSI Overdrive will do it.

To underclock the graphic card in OSX is much more complicated than under Windows.

1st of all, the card's ROM must be unlocked. Otherwise, it's impossible to underclock that card in OSX.

Then, you have to find out all the underclock setting (voltage, clock speed, etc) in Windows.

Dump the ROM image, edit it, and then flash the new ROM image back to your card.

This is not an easy task. It may brick your card. You may lost the EFI part (no boot screen until OSX loaded). It may even damage your motherboard (e.g. A wrong power setting cause it to draw too much power and burn the motherboard).

Therefore, it's not recommended unless you really know what you are doing and fully prepared for it.

Even though it can increase the stability. That problem may come back in few months and you have to further lower the clock speed or increase the voltage.

Anyway, your screen captures make me suggest that both the GPU and the VRAM has protential problem. Replace the whole graphic card may be the only solution.