need help with my check engin light!!!!

i have a 99 civic ex and i swapped catalytc converters with my friend(same year and model). its a gutted cat so when i put it on my check engine light came on... my cat has an O2 sensor on it so i dont kow if thats the problem. i diconected my battery and that didnt work the light just went back on 2 days later... i tried the backup fuse... that didnt work... somone please help me... i like the cat... it gives me a lot of top end