I recently got a black 2003 Mazda Protege and was surprised to find that the catalytic converter was built into the bottom of the header w/ sensors and all.. I already have changed the intake system and want to swap in a new exhaust system head to tail including the headers(of course). My problem is I was told that switching out the stock headers could result in my check engine light always being on.. Is this true? and if so how can I change out my headers and exhaust? would i have get new sensors too?

 

You can do whatever you want, you just have to rewire the O2 sensors so they can reach to the new location. The cat is not "integral" with the headers.

 

look at the position of the o2 in question. if it is between the catalytic converter and where the exhaust comes out of your engine then whatever you do it must remain before the primary catalytic converter and yes you do have to put a cat back in in order for the vehicle to run as clean as it was designed and therefor not throw the check engine light on all the time. If you're like me in New Hampshire you won't be able to easily get a sticker with that problem.

most after market exhaust systems should have a spot for the o2 sensor where you'll need it unless it's "not street legal". If there isn't a spot for the sensor a hole would have to be made somewhere before the cat in a place where it will get a good reading and a bung (big nut for the O2 to thread into) would have to be welded on at that hole)