I have a 2004 eclipse gs. I just installed a header this weekend and the Service Engine Soon light came on. I went to Autozone and they said the code was P0421 and I probably had a bad O2 sensor. I replaced the O2 sensor and i still have the same problem. Does anybody know how to reset the computer or have any suggestions on how to fix this problem? I went to the dealership and they told me that it would cost $110 to reset the computer and they couldn't gaurantee that it wouldn't come back on.

Disconnect the battery and go have a sandwich watch a little TV go back out side and reconnect it.

That seemed to fix the problem. Thanks for your help.

Will that clear the computer making it have to find the problem again or simply turn off the light, I was having a recurring cel problem, and that seemed to only turn off the light for about 25-50 miles, where as using the obdII checker thingy (exactly what it's called btw) it seemed to go about 150 miles until the light came back. Might have been a coincidence, I have no idea, I'm asking not trying to use evidence to refute you.

The problem is you probably have a cat-back (no catalytic converters) exhaust. If you dont have the cats, your car can throw codes because it thinks something is wrong. Does the dealer know you have a custom exhaust?

Try putting the old exhaust back on if you can. If the check engine light goes off then its the new exhaust's fault. i would weld the old cat(s) on the new exhaust but i dont think you would want to do that. i dont think theres anything wrong with your car, i think your car is just "confused"

So what i would do is put the old exhaust on and see if it throws a code

that will only make all the systems read "not ready," even if the check engine doesnt come on, but after the computer scans all the systems the code will reappear.

Disconnecting the battery would be like plugging a leak with paper: only temporary

I don't have a catback exhaust yet. I have only put a header on which bolted directly from the head to the downpipe. The header already had a place on it to put the O2 sensor in. It is not the O2 because i bought a new one and had the same problem. I tried unplugging the battery for an hour and so far ao good. If the light comes back on, I'll post it up here. Thanks for your help though.

Danman was right. My light came back on 32 miles after I restarted it. I'm thinking it might have something to do with the air/fuel ratio the header creates. Does anybody know anything about 02 simulators?

If you poke around there is a little mod that you can do with some spark plug piece that will offset the o2 sensor into cleaner air so it won't throw a code

I ended up buying a spark plug anti fouler that was already modified and I installed it on the O2 sensor close to the catalytic convertor. I have drove for 150 miles now with no Service Engine Soon light. If the light comes back on I will let everybody know but it seems to be fixed for now. Thanks for everybody's help.

what header is it, what brand. did you remove the stock cat

Brand of header makes no difference.