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Model A1225 / Mid 2007 and Early 2008 / 2.4, 2.8, or 3.06 GHz Core 2 Duo processor

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Won't complete start up process?

My iMac start showing black squares on its screen and then had distorted pixels on the display. It eventually got to the point where it will attempt to start up (bongs, shows apple icon and progress bar, but then goes to restarting in a continuous cycle. The start up screen is sharp and clear; it just can't complete start up.

I opened the computer up to see what the diagnostic LED's said. The four LED's are all lit up when the computer is plugged in and attempting to start up. LED's 3 and 4 go off when the computer restarts. I first thought the video card was bad but LED 3 is on, indicating that the computer and video card are working properly. Any ideas on what might be wrong?

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The problem was caused by a defective/corrupted hard drive. However, after installing a new hard drive, the original problem is gone, but I am unable to get the computer to boot from another disc or device in order to install the operating system (mac OS El Capitan).

I tried doing it using a time machine back up drive; a formatted USB flash drive with macOS El Capitan installer on it; and by trying to connect to the internet to download the OS. Nothing has succeeded. Ideas?

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Unplug the LCD cable and plug it into a moniter. If it has the same screen issues, then it might be the GPU

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6 Comments:

I completely agree. The main problem here is that the OP has diagnosed the problem and probably won't until he becomes totally frustrated.

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Yeah sadly this is most likely going to happen.. It's a classic case of bad GPU. If it was caused by a bad hard drive OS would install fine and boot up.

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OK, but why is the 3rd diagnostic LED lit if the video card is bad? I don't have an external monitor that I can connect to the computer.

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I can't seem to find instructions for replacing the GPU on the iFixit site?

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Oh my bad. I didn't know that you replaced the HD

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