Error 003 occurs when a System Menu 4.2 or higher from a different region gets installed onto a Korean Wii.
Error 003 Wiis can only boot in region game discs from the recovery menu from a SaveMii Dongle. Due to the enhanced region blocks of system menu 4.2/4.3, it is not possible to update these error 003 4.2/4.3U/J/E Wiis to 4.3K and there is no known disc with 4.3K update on it. One way to fix a 003 error Wii is to replace IOS70/80 with IOS60-v6174 installed into IOS slot 70/80. Installing IOS60-v6174 into IOS slot 70/80 will remove the Korean key check found in IOS70/80, thus fixing the Wii.
Currently all USA/PAL error 003 Wiis can be fixed by loading Super Smash Bros. Brawl from the recovery menu, running Smash Stack PAL then installing IOS60-v6174 into IOS slot 70/80.
Due to the enhanced region checks of system menu 4.2/4.3, Super Smash Bros. Brawl cannot be directly loaded from the system menu. However these region checks can be bypassed by loading an in region game disc, then swapping the disc to SSBB-PAL and restarting the Wii. There are a variety of methods depending on the game used; some games work, many games do not work.
All error 003 Wiis 4.2/4.3U/E are 100% fixable providing you have a modchip. Disc swapping methods utilizing "Mario Kart-PWNS" and DiscSwap-PWNS" will fix the issue. Unfortunately SSBB-PAL/Kor will not load on a Wii with the Japanese language setting, so SSBB can not be used as SSBB-Jap will not autoboot.
If SSBB cannot be used to fix error 003, the only other solution is to load an IOS used by a game that has the Trucha bug present, then disc swap to an app to fix the Wii using that IOS. If a NAND backup was made with BootMii or the NAND keys where acquired prior to the brick, NAND programming can also be used to fix this error.
Once homebrew is loaded, KoreanKii can be run to remove the Korean Key and boot the outside region's System Menu normally, or NUS Downloader can be used to get the Korean System Menu, and DOP-Mii can install this menu.