Hi,
My Mid-2011- 27" i7 iMac graphics card failed and Apple replaced it at n/c.... then 2 yrs later the replacement failed - they wanted $900 to replace it.... I really like my iMac (I had added a 4TB HDD and an SSD)... so I went on ebay and found a used one for <$300 and spent a lot of time on You Tube looking and reading and finding the "special thermal grease" from Greece. It takes time patience, static protection, lots of paper towels, some lighter fluid to clean off the old thermal paste... and luck. It took me a couple days to finish, and it lives again. I also read that the apple fan control system is more for sound than safety - I use Macs Fan Control now and am happy with a noisy/cool iMac to be able to process my work file.