Chosen Solution
I have been scouring the internet trying to find any information on my issue, but I am at a loss. My iMac turns on normally, I hear the chime, starts to load the progress bar, gets near the end, and then the screen just goes blank white. It does this trying to boot from DVD, countless USB installers, Internet Recovery, and other HDDs that I have tried installing. I have tested with multiple known working RAM chips with no change. The thing will just not boot up from anything. I would appreciate any help and I will try any advice in a timely fashion and let you know the results. Thank you.
Most likely failure is the GPU. When it fails you may see it boot and the progress bar go to about 65-70% then it may shut down or shut down and reboot. I have one on the bench right now with the same issue. This is a known issue and there was a repair program on it for a while. https://9to5mac.com/2013/08/16/apple-ope… On mine I intend to do a reflow of the GPU with a hot air workstation. I have had about a 70% success rate doing that. The replacement GPU are still running in the $200-$300 range because of high demand. @ danj knows of some replacements you can try. Personally I am looking at the Dell Alienware M17x R5 Ranger nVidia GTX 765M 2GB Laptop Video Card 9R3F5 There are some videos I am watching to learn how to make it Mac usable but I have not finished yet. I emailed this distributor yesterday to ask if this one was already flashed since he stated it is for the iMac and costs about $30 more that the others: https://www.ebay.com/itm/New-Nvidia-GeFo… I will keep you updated on my progress if you want. iFixit does not have a full guide on replacing this but this guide will get you close, I may take some pictures to do a guide while I am at it: Installing iMac Intel 27" EMC 2429 Dual Drive Kit (HDD or SSD)
Have you reset SMC and PRAM? This model has a known defective GPU also. @dan ‘s request to attach an external monitor will tell you if this is the culprit. Inside the iMac there are 4 diagnostic LEDs. Before you go that far start it in safe mode and see what the verbose readout is. It may give you your needed information.
