returning my car to stock so that I might sell it in the future and I had the stock header put back on. everything was fine except what seemed like a lower than usual idle.

drove the car for a good couple of days and on a late night drive to white castle the check engine light came on out of nowhere.

no problems driving at all, idle is still somewhat low ( in the 600-700 rpm range) but you'd never know anything was wrong with the car unless you saw the CEL.

I had the code read and its a cylinder one misfire. i cant think of any reason why it would come on. could it possibly be a faulty o2 sensor?

i suspect there might be an exhaust leak because i can smell fumes when standing beside my car at the door...i havent gotten under the car yet to check though. that have anything to do with it?

cliffs: reinstalled stock header and CEL came on two or so days later. why?

 

Fix the leak.
Reset ECU.