keep the 2nd one because its the one that does most of the work. the first one is just basically for cleaner start ups. since its closer to the front, it heats up faster than just one in the traditional spot. to me, the first one is totally useless because without it, it makes no difference after 5 minutes or so. i guess nissan is putting us up against a honda civic dx....hahaha.Kevin C.---'01 SR20DE said:Where are the O2 sensors? Is there one after the second cat? If not, i'm ditching it an keeping the first one.