Chosen Solution
Is Apples diagnostic suite free for iMac 2008 that’s out of warranty? I mean at the Apple store. Update (01/09/2018) @danj @captainsnowball Here is a video of it:
So when I turn it on it has all but one LED lit that’s the 4th led for display. After a couple seconds it automatically restarts cause the display isn’t responding. Then it just stays like that until I turn it off. P.S at first uploading this video it took about 10 minutes. Then it says video limit of 30sec. Then I had to edit my video and upload it again!
@captainsnowball is this what you wanted? The title of your question undermines what you really want and that is to know what’s wrong, then how to fix it. First, when it is booted up can you discern the desktop, (use a flashlight held at an angle to the screen). If it is there, replace the inverter. Here’s how: iMac Intel 20" EMC 2133 and 2210 Inverter Replacement UPDATE If you can’t discern the desktop, hook it up to an external monitor. If that fails it narrows it down to the GPU.
Your original grey disks has the diagnostics on it. Here’s a bit more: OS X El Capitan: Use Apple Diagnostics or Apple Hardware Test
