iMac 27" Late 2012 Not Booting up

[vaxes]

Hi

My iMac died on me last week.
It's been a bit flakey for a couple of years but has always come back.
I have 3tb hybrid drive (this was replace by apple about 7/8 years ago due to a hard drive failure)

Now every time I try to log in the iMac crashes and reboots.

I can't get it to boot into safe mode, recovery mode, I have Reset SMC, I have Reset NVRAM or PRAM and It won't load a OS usb stick.
Safe mode, recovery mode and Booting up with a OS USB stick all get to about 50% on the apple logo and the get stuck there.

I can get onto Single-User Mode but this stalls when trying to run any diagnostics.
I can run target mode, the iMac hard drive comes up and I can pull content from the iMac with no issues.

I am at a bit of a loss as to what to try next and hoping someone has any ideas?

Thanks

 

[Juicy Box]

Booting up with a OS USB stick all get to about 50% on the apple logo and the get stuck there.

Could be a few things, logic board failure, GPU failure, or maybe the Fusion Drive.

Failing internal drives can sometimes cause boot issues, even when booting from a separate external drive.

 

[vaxes]

Sounds a little more complicated then I was hoping it would be.
Is there anything I can do to narrow down what it could be?

I was hoping because target mode worked, that the hard drive might not be a complete write off.

 

[Fishrrman]

If it will boot into safe mode (and by "boot", I mean boot to a login screen and then the finder), that seems to be indicating a failure of the discrete graphics (assuming it has that).

Reason:
A safe boot "bypasses" the discrete graphics card and uses the CPU's "integrated graphics" instead.

This might also indicate why, when you run diagnostics, it fails -- because when the diagnostics tries to access the discrete graphics -- it can't.

It sounds like the internal fusion drive is still functional.

VERY IMPORTANT QUESTION:
Do you have a backup?

My advice:
Bite the bullet
Make sure you're backed up
Don't put any money into a 12-year-old iMac
Start shopping for a replacement.

I'd suggest a 2023 Apple-refurbished Mac Mini m2pro model, along with a 27" 4k display.

 

[Bigwaff]

Booting up with a OS USB stick all get to about 50% on the apple logo and the get stuck there.

What version of Mac OS is your "OS USB stick"? 2012 iMac supports Mac OS X 10.8 Mountain Lion - 10.15 Catalina.