i have an 03 evo viii. the only mods i have are a turbo back exhaust, intake, and a manual boost controller(set to 18-19lbs). my check engine light came on about three hours after the install of the turbo back exhaust. everything still seemed to run fine. so i took the car to a friends shop to see what the cel was from. the cel is from the exhaust. i noticed that when i get on it a little bit, it feels like the plugs are fouling. so i changed the plugs thinking that might help. it did for a day or so, then it started running like crap again. judging by the old plugs, it seems to be running lean. could the cel be causing this? any thoughts would be awesome. thanks.

 

If you switched out the cat for a test pipe your CEL will come on. The computer is thinking that the cat is bad because it is not sensing a change in O2 between the 2 sensors.
Search for spark plug non fouler and you will find a $3 fix for this problem.

 

+1

Or getting a tune is always an idea! Try looking for a tuner in your local area that will do a simple street tune for you, they arent as expensive and i dont think DYNO time is needed nor worth it over a street tune for just simple bolt-ons.

A tune will get rid of that CEL and trick your ECU into thinking everything is A-Okay!