iMac 27", late 2012 reboot loop after chime, no AHT no recovery

[giuarb]

Hello all, hope I can get some feedback here.
My glorious iMac is basically stuck in a loop, power on, chime kicks in, apple logo with progress bar then reboot and back to chime etc etc., all clue bring to the fusion drive interfering and prevent boot from other sources. Note that this 3TB fusion was replaced during the recall by apple.

Things I have tried:
UPDATED

1) Reset PRAM and NVRAM several times , no difference
2) SMC reset, unplugged for a minute, plug back, no difference
3) Tried to access recovery and internet recovery, same story, progress bar to approx 50% then reboot, for internet recovery, it actually shows the globe but after a few minutes of progress bar it reboots.
4) Tried to boot from USB (holding shift works and brings up the selection menu with wifi etc etc, keyboard and mouse work properly), after selecting the drive, like my time machine or my usb Mojave installation, about a minute of progress bar and then reboot
5) Put back 2x4gb original ram, no difference
6) Tried in single user to run fsck -fy and I get a drive error
7) Tried TDM, pressing T at boot, the computer seems to go in drive mode so I may be able to connect to my laptop with a thunderbolt which I don't have at the moment.

Here seems that the fusion drive prevents the machine to do any boot from other devices, what I don't accept is that I can't run the diagnostic which should be something really strange, thats why I have asked help here.

I am keeping this post alive since may help other users in the future

Only once at the begin I did receive an EFI error
Error:8000000000000003, Cannot load 'EFI/Drivers/TestSupport.efi'
Status: 0x00000003

Will try next https://github.com/upekkha/AppleHardwareTest

Any suggestion?

 

[priitv8]

Boot in verbose mode to see actual activities?

 

[Fishrrman]

Have you ever changed out the RAM from factory-original?

 

[nambuccaheadsau]

If you want AHT go to this link, choose the precise model, download. Burn to a USB thumb drive and boot from it. Let full test run and it will check memory etc.

GitHub - upekkha/AppleHardwareTest: Apple Hardware Test Download Links

Apple Hardware Test Download Links. Contribute to upekkha/AppleHardwareTest development by creating an account on GitHub.

github.com

 

[giuarb]

I did but, I will try to capture the results since it reboots after about 10 seconds

 

[giuarb]

Yes I did but it reboots after few seconds but single user works and I have tried to check the disk via fsck -fy, I am attaching a picture, it seems the fusion drive is the problem but why I can't boot from the time machine or from an usb key?.
Also tried to swap the ram (put back the original 2x4gb)

Boot in verbose mode to see actual activities?

 

[priitv8]

Looks like your boot fusion drive is bust.

 

[giuarb]

that’s what I suspect too but I don’t understand why I can’t boot from USB and why I can’t start recover or disk utility) and of course why reboots on its own!
TDM seems to work (I get the symbol on the screen but I don’t have a thunderbolt cable to try)
).
funsion drive was replaced under warranty under a recall.
anything else I can try? I know I can bring it to the genius bar but I don’t give up easy, there must be something to try

Looks like your boot fusion drive is bust.

 

[Fishrrman]

OP:

I may have missed it above, but do you have (or can you borrow) an EXTERNAL USB drive that is "fully bootable to the finder"?

If you attach that to the iMac, then try to boot from it, what happens?

Even with totally messed-up or failed internal drives, it should still be possible to boot the Mac from an external, bootable drive.
If you cannot, then it points to some other kind of internal hardware problem that is probably not "drive-related".

 

[giuarb]

External USB, tried, still reboots.
I am sure that If I open and disconnect the fusion, it will boot from external, I will order the adhesives.
Also If I have to pay apple to replace the fusion, I will rather install 2 SSD myself.

 

[giuarb]

It is common that a fusion prevents the machine from booting from anything else?