Getting headers an custom exhaust done, going to need a bung made for the second O2 sensor, will it need to go after the cat or before? Dont want to throw any cels.
but after I do will the typical ECU reset work?

 

After the cat.
Just keep O2 sensor #1 before the cat & O2 sensor #2 after the cat & you should be ok. wink

 

lst o2 sensor goes before the cat, it tells the ecm how rich or lean the car is

2nd o2 sensor goes after the cat, it tells the smog nazi's how clean your car is.