The following are known Action Replay Codes for Need for Speed Underground 2 on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
856fd470
b8dc5efa
5681c520
de4dcfae

Always Have Low Time
0531affd
a518ed9a

Infinite Nitrous
413d34bc
94c0fde3
8e883eff
92e9660d

Infinite Purchase Points
371e6572
8d4f7384

Have All Races Open
c31afeed
e787dc92
0609f3da
e806d0f7

Have All Cars/Parts Open
5bb78695
d2744f07
9256468c
38df6fca

Have All Open Cars Owned
ff8360d2
7925411f