Bushing has created a disc to fix NTSC-U wii's which got semi-bricked by the Mario Kart updates or any other out of region updates.
Download: Semi-brick Fix (NTSC) Disc for Wii
To quote: I've made a disc which will hopefully fix any NTSC Wii that got bricked by an imported Mario Kart game (or similar). It is an ISO with one partition -- an update partition -- and it contains three files: IOS30-64-v1040.wad, SystemMenu-v289.wad, and __update.inf:
System Menu v289 is the newest NTSC version of the system menu available on Nintendo's download servers; it's what you will get if you go into the Settings Menu and perform an update. I packed it into a WAD, and maintained the original, valid Nintendo signature, so it should be safe to install. (The disc, however, is fakesigned. This should not be a problem.)
I'm including IOS30 as well, just in case -- it's what this system menu uses. I would expect all systems out there to already have it installed, but hey.
Obligatory warning -- I have tested this on 3 Wiis and it seemed to work fine, but I can't make any promises that it won't brick your Wii. It contains no safeguards; use this ONLY on an NTSC (US) Wii. If you use it on an NTSC/J or Pal Wii, it WILL BRICK you. (If there is a need, I can make similar discs for those systems.)
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