iMac 5K, 27" Late 2015 - A1419 (EMC 2834)

[yobad]

I have an iMac 2015 Serial number : DCPS207EH3GT, stuck on apple logo when starting, with half of the loading bar loaded.
I didn't open it yet, but it could be GPU issue.
I tried to boot form a second HDD, but it is the same issue.
I don't know the motherboard part number, how to check it before opening iMac ?
Is this iMac model know for this issue?
Thank for your help.

 

[2informaticos]

Test booting in safe mode.

 

[yobad]

I tried to find what commande to entre safe mode Command+ R. It is not working.
I need a wire keyboard ?
Is it correct?
Thanks

 

[2informaticos]

Hold down Shift and press power button to start; maintain Shift key pressed, untill Apple logo appears.

 

[yobad]

I tried safemode, still the same issue.
Don't boot on os, stuck on apple logo and loading bar, once I successfully booted macOs installation.

 

[2informaticos]

Then you must to work on the MLB.
Known good BIOS clean ME is worth to try.

 

[ipetro]

I tried to find what commande to entre safe mode Command+ R. It is not working.
I need a wire keyboard ?
Is it correct?
Thanks

The keyboard must be connected via cable to the iMac.