iMac 2011 Display Issues

[DarkMatt]

Hi everyone! First time posting is to ask for help - thanks for your patience!!

I've been a MacBook user (and am still loving my 2016 13" TB) but was recently given a Mid-2011 iMac. I need some advice on tracking down the issue. When I press power, I'm greeted with the normal chime, and then the screen comes on, and I immediately get distortion/lines/wavering/flickering across the screen. At first I suspected it was likely to be an AMD graphics card failure (I tried plugging into an external monitor and didn't get anything) - but I've not seen a graphics card failure like that before - it seemed more like an LCD failure.
Any advice? Does it look like a GPU failure, or more like an LCD issue? If it's a GPU issue, can I disable the GPU and use the integrated graphics? If it's an LCD issue, should I be able to use an external monitor from boot, and is there a trick to doing that?

I've attached an image - that's the recovery logo on the screen (just with much more wavering in person!). I can get it to boot into Linux from USB also - but the issues do persist.

Thanks everyone - I appreciate your time and help in advance!!

 

[redheeler]

The simplest way to confirm a GPU failure, if the Mac is still bootable, is by taking a screenshot. If the distortion appears in the screenshot, it's caused by the GPU for sure.
[doublepost=1508637466][/doublepost]As for the integrated graphics, I don't think that will be an option as Apple has disabled it in the iMacs that have dedicated graphics. The best you can do is delete the Radeon kexts from MacOS and force software rendering.

 

[dcmaccam]

My Mid 2011 iMac 27" has a AMD Radeon HD 6970M 1 GB card. This failed about 12 months ago with a similar problem. The machine came with a 3 year warranty but it was about a year out of date when it failed. However the card got replaced by the retailer under UK consumer laws as there was a known problem with this graphics card. If I had to pay the repair cost it would have been about £600.

I believe the problem was solder ball failure on the GPU chip. Some people had some success with getting the chip removed and re-balled then re-attached.