Chosen Solution
I just replaced my logic board in my 27" imac after I broke off the display port connector on my original logic board. It booted the first time I tried, but the fans were on constantly and I forgot to reattach the wifi cable so I took it apart again. Then I booted I without the display installed to see if the fans were running normally. They were so I powered it down by holding down the power button. I then installed the display and proceeded to boot it up again. I cannot get it to boot now. It is stuck on the apple logo with white background and the loading bar moves slowly towards the end but it never leaves this screen. I have tried SMC reset, pram reset, safe boot, boot to recovery partition, booting from external USB drive, booting to disk utility. How can I fix this? Why did it boot the first time and now it won’t? Update 1: I was able to boot up in OS X after I reinstalled the video card. It seemed to work fine. Then it froze up today. Now it won’t boot into OS X anymore. I was able to run the Apple Hardware Test and it said there was not problem with the hardware. How do I test to see if it is the video card causing the problem?
Next time contact us first, Apple would probably have fixed it for free under this program: iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program https://support.apple.com/en-us/HT203787
Hmm, thats normally what happens when the GPU is bad. Try to reattach the graphic card. What does the LED says?
Seems like a problem with the video card. As you know video card is separate from logicboard. Another possibility might be the lcd cable. Check it out. Thanks.
