I'll try and make this short...dreaded P0420 code, O2 sensors, etc. ruled out. California is crazy, as most of you know by now, and so there are restrictions on aftermarket cats. To bring you up to speed (though it's a little confusing), there's this post on Yotatech: 2000 4Runner CA catalytic converter mystery explained | Catco vs. Miller?? - YotaTech Forums Even though that's from 2yrs ago, the long and the short of it is that the 2000 4Runner 3.4L from the engine family YTYXT03.4FFR has not been added to the "certified" list supplied by Airtek/CatCo, and thus there is no exemption for those cats to be used on the 2000 4Runner, even though the SAME ENGINE FAMILY is approved for the 2000 Tundra. So what you're left with as an owner of a 2000 4Runner in need of a new cat, is a minimum $850+ price tag for the cat (if you talk sweet to Cruiserdan--I think they sell Toyota authorized stuff), plus your labor or paying for someone else's. According to that Yotatech post, the CA Air Resources Board folks say it could be as easy as Airtek updating their lists and telling ARB about it...but they haven't. So there's that.