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