Just posted on this subject in the other thread you wrote in...here is what I said...
Hey Yukute, I have a Cali 4runner in Colorado too. Was getting codes after having it for a year or so (140-150k miles) and looked into this. Found out that Colorado is really astute with their testing so decided to keep it stock. Found the correct front sensor (A/F sensor) online for $80 bucks. Cleared all the codes for about three months, then started up again...started to run 91 octane and did a couple good fuel cleaners and have been great ever since.....IMHO its hard to get past keeping the exhaust stock here unless you want to spend the $$$$. Good luck
2000 4runner Ltd-Resonator box delete, deck plate mod, OME 3” suspension lift (881s w/ top plates and 890s), Toytech center diff drop, rear diff breather mod, 265/75/16 Geolander AT/Ss, Rola Vortex roof rack, Toyota option brush guard