I just bought an other Wii U system with Black screen from the same guy. Do u guys think that the Black screen might be a CBHC Problem? Should i open a new Thread for this? ^^
There's very few ways you can brick a Wii U, so it's either one of these three cases (someone correct me if I'm wrong anywhere though):
A)Coldboot Haxchi brick
This one is the most common afaik, and it's due to user error. It happens when the user installs coldboot haxchi (cbhc) over a non-legit (pirated/non-legal) DS VC game.
Since CBHC "replaces" your sistem menu with the DS VC game, and your game needs signature patches to boot because it's a pirated game then your Wii U displays an error because it can't access it's "home menu" (now the DS VC game) on boot.
B)Brick by deleting critical files
This one is also because of user error.
This happens when the user deletes important files of the NAND (such as System Menu files, for example) with FTPiiU everywhere, leading to a brick because the system can't find and load the files it needs.
C)Destroying the motherboard
This one is pretty self-explanatory, if you break the motherboard or part of it then your Wii U is bricked.
A and B can be recovered via hardmod if you have a backup of your OTP and/or your clean (not modified/pre-hack) SLC, you can't fix C for obvious reasons.
I hope this cleared some doubts you may have about what kind of bricks there are and how to fix them
EDIT: There's also some reports of bricks due to random corruptions, but they are not really common and since nobody knows what caused them there's not much documentation about them, but I think they're also recoverable via hardmod + OTP/SLC backups
