help me? advice? anyone?? needed desperetly!!

ok, i have been working on my friends 2003 Lancer OZ rally. It has been a head ache... he complains a bit about perfection so this is why i am asking for anyones help... THe check engine light came on when i installed his K&N cool air intake on his NA motor.. it will not go off, i have tried everything that i know! I am in the dirt with that little issue. THEN! his idle got messed, i didnt even touch the idle control bolts, and the ilde fluctuates so much and so rapidly from 500 rpm to 1500rpm, what is up with that??? i am so confused.. i also installed a full ractive exaust system. the header is going in next week.. but he says that he has lost horse power?? how can i fix this??? also? anyone have any ideas to what i can do, $$$ is no object for this guy and he wants it just to run perfectly, so is there something i missed... ??? email me if you have ANY, and i mean ANY advise... thank you!!! [email protected], or my ICQ is 260-367-186 :)