Level select:
Hold Run and press Select three times when the house and lightning are displayed on the title screen. Then hold Down/Left and press I. Note: To face the final Boss, select level 0.

Level select (Japanese version):
Press Select three times, then hold Left and press Run when the house and lightning are displayed on the title screen. Note: To face the final Boss, select level 0.

Sound test:
Enable the "Level select" code. Then, press Select at the level selection screen.

Expert mode:
Hold Select at the title screen until the word "Hard" is displayed.