Youkai Douchuuki (NES)
A port of the arcade version, Youkai Douchuuki is probably best known for its unforgiving difficulty and catchy main theme.
Hold Select on the first controller and press Reset. The game's title "妖怪道中記" ("Youkai Douchuuki" or "Demon Travel Journal") will be changed to "妖艶道中記" ("Youen Douchuuki" or "Erotic Travel Journal").
Hold Up + A + B on the second controller and press Reset. Press Left or Right to select between "OBJ", "BGM" and "EFFECT" test modes. Press Up or Down to change the current test object or sound. Press A or B at the "BGM" or "EFFECT" options to start or stop music respectively.
The Game Genie code KXESLKNP re-enables "SCENE SELECT MODE". To enter it, at the title menu, enter the "CONTINUE" option. Another "CONTINUE" menu will appear with the extra option "SCENE SELECT MODE". The first three options stand for "STAGE", "SCENE" and "FLOOR" that you can select to start the game with. The two last options are the same as "BGM" and "EFFECT" in Test Mode.