adding a header
 

would adding a header without a tune with the stock cat make the car run lean, and can u add a test pipe with a o2 bung so u dont throw a code and not run lean or do u need tunes for that.

Adding a header alone will not make the car run lean. Most people typically don't run into that issue until they have a full header, test pipe, and exhaust. You could throw it on a dyno at that point to check the a/f at that point. As always, some sort of tuning option is best with modifications.

definitely not an issue on ap1's as they run on the rich side at WOT, some ap2's ran a bit leaner in 06+ so I'm not sure how they might be affected.

i have a 04 i wanna add a skunk alpha header

my 07 ran pretty lean when i had my i/h/e setup. tune made it all better tho. guess it really depends on the MY you have.

An intake, header, test pipe, and exhaust made my 2004 run lean and all over the place at WOT. I don't know AFR's prior to these mods as I didn't install the WBO2 until after all these were in place. It would hit over 14:1 at WOT.