Exactly its located on the header right before it meets the downpipe. the rear o2 is after the cat. there IS a spot just before the cat that satisfies the wideband requirements. In theory you need both the because if something goes inside your cat you would know because of an engine code. The only time you throw 2 or 3 codes is when you swap your header and downpipe. Its telling you theres a shitload more cfm moving by the rear o2. when you tune you change those to not report