I installed a apexi gt header and a greddy evo cat back exhaust. It set off my check engine light like I knew it would, but I have tried to reset it and something keeps setting it off. I have a 2000 Honda Civic Ex.

 

always reset the ecu after any engine mods..... to make sure engine reads the new readings as new and wont trow codes

and BTW u are in the wrong forum

 

You said header... check your O2 sensor.