well, il explain, u have to start the level from tyhe very beginining on normal or higher difficulty, not on easy or from a mission rally point. the the corect code is 4 6 5 4 5 3 4 although i used the extended code which also works. and make sure u kill all the flood AFTER THE CUTSCENE (where u have to battle the flood right at the end to escape) fro more info:
http://halo.wikia.com/wiki/Halo_3_Skulls
HOPE THIS IS ENOUGH TO ANSWER YOUR QUESTION