Chosen Solution
I have a 24" Mid 2007 A1225 EMC 2134 that gives me a POST tone, 3 LED lights then shuts down and restarts. It then hangs on restart at the 3rd LED. Completely black screen. No signal to external monitor. The hard drive checks out on another machine and has 10.9.1 installed. I am finally just going to pay Apple support to see if I can get a correct answer before buying any parts. Has anyone had any luck yet? From this question a fix would seem to be reballing the GPU: No Backlight No Display 3 Diag Lights on eBay has a guy who is doing these for about $100. This will also answer this question: Faulty iMac - Only three LEDs appearing in diagnotics? We really need an answer for this one instead of replacing the video card or the logic board. Both parts are still high dollar if we’re just guessing.
Good luck trying to get any help from Apple since it sounds like you have taken apart the computer already and you’re out of warranty. In my case, my hard drive was running fine in another machine so I tried to torch the chip to reball it myself and that failed. I figured I didn’t have anything to lose at that point. Since that didn’t work I ended up sending the card with the bad GPU chip to Brickfence on Ebay and having the card replaced with a new chip. Brickfence told me both the chip and card were bad. I had them replace both items since you can not find new cards any longer. Their products came with a warranty and the service was great. I got a replacement card and chip for $349. Much less than replacing the machine.
It to me seems to be the video card as the most logical problem it seems to be that the video card is either not picking up the screen or is corrupted itself or the driver for it has been corrupted I recognize this issue vaguely from a different computer I had worked on my bet is that the video card has been corrupted. So you may have to either buy another video card or find a way to check the drivers because one of them may be corrupted as well.
