iMac 2011 booting into grey screen

[r6mile]

Hi,
I've recently procured a 21.5 2011 iMac with a faulty GPU from eBay. I've got a spare GPU waiting to be installed, but I wanted to make sure there's no other issues with it before I open it up.

However, I can't seem to get it to boot, either from the 10.13 recovery partition on the HD (which will be replaced by an SSD), or from my external USB drive with a bootable High Sierra install (cloned from my personal iMac).
The computer goes through the loading bar, but then either gets stuck there for hours and hours and never finishes, or it just eventually leaves the loading bar halfway and moves to a grey screen (where it never moves on).
PRAM and SMC resets did not work. Safe Boot didn't work (just moved to like a 'forbidden' symbol). Is this a firmware issue? Any ideas on how to fix it very welcome please!

Thanks,

 

[chscag]

Is this a firmware issue? Any ideas on how to fix it very welcome please!

I don't think so. I've run into other users with the same problem which was resolved after the replacement GPU was installed and working. I know it's a lot of work to replace that bad GPU and then find out you have the same problem. But I really don't see that you have a choice. Let us know how it went.

 

[ScreenSavers]

A gray screen on boot up on any Mac from 2011 is definitely the sign of a bad GPU. You wont be able to get it booted until its repaired or replaced.

 

[r6mile]

Thank you! Good to know it's probably the GPU (which I know is faulty already). I will let you know the result once it's replaced!