Stuck on white screen at boot up

I'm trying to get this iMac back on it's feet and I'm guessing I'll need to buy some parts. But my problem is I need some kind of diagnostic check to narrow down what parts I actually need.

The problem I have is that on boot up the iMac gets stuck on the blank white screen immediately after the blank blue screen.

I've tried the following:

Safe Boot-hold down the shift key while booting, key must be held down till the gear shows up under the apple:-

This allowed me access to the OS and I repaired the disk but on normal boot up it got stuck on the white screen.

Boot to the Install DvD and run repair disk (holding C down during boot):-

Would not find the start up disk and stuck on the white screen.

Hold Command, Option, P and R during the boot process to reset NVRAM:-

Done twice but same result on boot up with white screen.

Now I'm out of options. I can't start spending hundreds on spare parts to see which is the offending party.

Any help much appreciated.

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Solution retenue

You've already began some good diagnosis.

I think you have a bad or failing HD. or, possibly a VGU problem. You should attempt to connect to an external monitor (if you see nothing, or, if you see a normal desktop it can point to possible solutions). If you see a normal desktop it means your problem with the white screen is internal display chain. If you see a white screen it would mean the VGU, or the HD is in the problem link.

After experimenting with the external monitor, depending on what you find, try removing the HD and booting to the repair disk... if no white screen it confirms bad HD. If still white screen you have other hardware issues.

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Thanks for your suggestion machead3.

I'll try and ask around for a spare monitor and then buy a mini-DVI cable.

par

I'm assuming a bad VGU is an immediate logic board replacement?

par

It may be possible to find a replacement VGU - though often it is easier to locate and replace the entire logic board. You can't use just any GPU/VGC it has to be one that was used in this machine, or there will be EFI/extension (AKA driver) issues.

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