The following are known Action Replay Codes for Mega Man Zero 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
c2497b68
d5b35da8
cbeedd47
7361f305

Infinite Health
57351cdb
9e6b4919

Infinite Credits
f35e7df4
713a3f28

Have All Elves
bd9319a1
43cfb07a
f7f9ee8a
2a20ae2c
d7753f0c
4e9a8c14
3fe12eae
7eb217cd
5c6a475c
ac3e30bb