Always win XG career mode races:
Press R, L, Z, L, R, Z, R, L, Z after ''Press Start'' appears at the title screen. A message will confirm correct code entry. This allows you to win a race no matter if you quit, lose, or die. Note: This effect lasts for only one race and must be re-enabled before the next one.
Source: EGM

Boost Start:
For a booster at the start of a race, push forward at the start of the countdown, then just as 'Go!' appears, press A, and you'll rocket off!

Double prize Money:
L,R,Z,L,R,Z,L+R at the start screen will give you double the bonus money.

Extreme Lap challenge:
L,R, L,R L,R, Z, L+R on the main menu.

Infinite Shields/Turbo:
At the Press Start screen, press L+R, Z, L+R, Z for Infinite Shields. This will also give you infinite turbo boosts.
Repeat to turn the code off.

Unlimited Ammunition:
Press L, R, L, R, L+R, Z
This will give you unlimited ammunition.

Unlock all Teams:
At the start screen enter L,L,R,R,Z,Z,L+R,Z

Unlock all Tracks/Teams:
At the start screen enter L,L,R,R,Z,Z,L+R+Z

Unlock Pion 6:
Complete the game on career mode to unlock Pion 6 in arcade.

Unlock Team Starcom:
To unlock Team Starcom, simply play XG Career mode all the way through to the end, winning the race on Pion 6.

Win current race:
L+R+Z, L+R, Z, L+R+Z you will win the race even if you blow up or quit