Chosen Solution
After attempting to swap out the GPU on my 2011 21.5 inch iMac, it refuses to boot. After plugging it in and pressing the power button the fans will spin up for a moment then stop for several seconds, this will repeat but nothing else happens. I’ve replaced the old (working) GPU and it’s no different. Any advice on what it might be? Thanks a million!
No LED diagnostics lights indicates no power to the logic board. Recheck your connections on PSU to logic board. On this repair I prefer to go with the 661-5969 661-5968 Video Card AMD HD 6970M 2GB For iMac 27" Mid 2011 A1312 Why? because the original cards just fail, getting a used one is a likely failure scenario. I get this one out of Hong Kong but right now it’s taking a month to get. I have not had one of these new cards fail to date. Cost is just under $200. https://www.ebay.com/itm/NEW-661-5969-66…
You need to check if the CPU is correctly in the socket and make sure you don’t have a bent pin in the socket. Did you properly load the pressure plate? Just like changing a tire you need to slowly increase the pressure evenly across all of the screws tighting them in a zigzag pattern. No diagnostic LED’s tells me you have a short, likely you mounted the chip incorrectly.
