All ATVs and tracks:  Enter Frog, Frog, Helmet, Speed Burst, Tire as a password. Note: The Speed Burst icon resembles an arrow pointing up.

Alternately, enter Wheel, Wheel, Lightning, Lightning, Speed Burst as a password.

Extra ATV's in Open mode
ATV open type one: Finish level 8 in Pro mode.
ATV open type two: Finish level 3.
ATV open type three: Finish level 6.

Extra ATV's in Stock mode
ATV Stock type one: Available from the beginning.
ATV Stock type two: Finish level 3.
ATV Stock type three: Finish level 6.

Passwords fo Challenge mode: Pro Difficulty
Lightening = L
ATV, looks like an X = X
Wheel = W
Helmet = H
Up Arrow = Up

Finished level 1 L H H X W
Finished level 2 W X L L Up
Finished level 3 + New ATV Up L H H L
Finished level 4 X W X Up H
Finished level 5 L Up H L X
Finished level 6 + New ATV W X X H W
Finished level 7 Up W W Up Up
Finished level 8 + Open Mode and ATV X H X L L

Passwords for Challenge mode: Open difficulty
Lightening = L
ATV, looks like an X = X
Wheel = W
Helmet = H
Up Arrow = Up

Finished level 1 L X W H H
Finished level 2 H L Up Up X
Finished level 3 + New ATV Up H W L W
Finished level 4 X Up Up W Up
Finished level 5 W L L X L
Finished level 6 + New ATV H W Up L H
Finished level 7 Up Up L W X
Finished level 8 X X H Up W
Finished the whole game (level 9) W W L L Up

Passwords for Challenge mode: Stock difficulty
Lightening = L
ATV, looks like an X = X
Wheel = W
Helmet = H
Up Arrow = Up

Finished level 1 L L X W L
Finished level 2 W H W H H
Finished level 3 + New ATV H Up Up Up X
Finished level 4 X L W L W
Finished level 5 L W Up H Up
Finished level 6 + New ATV W Up L X L
Finished level 7 H X Up L H
Finished level 8 + Pro Mode and ATV X W L W X

Gameshark codes

(m) Must Be On CEB81FA568312841   D798ECEB8C050A62
Infinite Time 6AC211AD83E6FD24   09F66FA228A5FDCC
Start On Lap 2 94C0FC62247A5968   85A9AC7BDBE066D7
Start On Lap 3 FD7FFCEF7F79D388   F4891DB4C3A4ACF2
Start On Lap 4 890B943A87EA44A3  407854B69EF4034A
Start On Lap 5 0A7691F5038BE73D  FF5A2E41ED2F7617