Mid 2014 Macbook Pro Retina - Black screen on boot, no chime I recently tried to install big sur on my macbook through a bootable USB drive. Everything went well until during the install the computer auto rebooted and showed the apple logo and load bar then went blank. I guessed the system was still installing so I waited 10 hours but still a blank screen. Now I have tried to unplug the USB and restart but the computer seems to be stuck in some kind of limbo. Here are the symptoms:

No chime when powering on
Backlight turns on but nothing is displayed
Fan spins
Ignores closing laptop lid (Fan still runs and backlight on)
No display signal (external displays say no signal)
USB gets powered on
Cannot boot into anything (Recovery, internet recovery, verbose mode, safe mode, usb drive)

I have tried:

Resetting SMC
Resetting NVRAM
Removing battery, leaving for an hour and reconnecting.
Replacing SSD with a working SSD from an identical macbook model. (still nothing)
Removing and wiping SSD using another computer
EDIT: I tried letting the computer sit with the screen blank to drain the battery as a last resort and now it will display the red low battery graphic if I press power on. But still no boot.

It seems base system is missing or corrupted. I have tried everything. What do you think?
It’s the I/o, disconnect it and it will work again. Or change the I/o board. Tried it and it worked! Good luck