Hello everyone,
So I have done a ton of searching on here for possible solution to my problem but so far haven't found one. I have a 2000 T4R 4WD w CA emissions. The story starts like this. About 6 months ago I was getting the P0420 code on and off. After reading through some threads I replaced the downstream 02 sensor with a Denso brand from Amazon. Code went a way for a while but then came back. Then I read to check for exhaust leaks as well. The flange just after the rear cat was indeed leaking. Had that fixed at a shop and haven't had the code come back yet. But now the truck is due for a smog check and I can't get my catalyst monitor to ready. I've done the drive cycle that Toyota put out in a TSB perfectly more than 5 times. Made sure all of the pre conditions were met each time. Torque Pro isn't showing any pending codes either. The upstream AF sensor has been replaced but not by me and I've owned the truck for 3 years. Should I start by replacing that sensor? Is it possible the downstream Denso one I ordered last year has gone bad already? Looking to check all the boxes before changing out the cat. The truck runs fine otherwise. No hesitating. No rattles from cat. Appreciate the help!

OP-What is your mileage?

I had the same issue in my 2000 4Runner. I was just a hair under 200k miles. Had similar codes, but cleared with new upstream and downstream sensors a few years back. Tried resetting the computer by disconnecting the battery overnight. Did the drive cycle many, many times. It never re-set. Went to my mechanic and he couldn't figure out a fix aside from new cats. Once that was done, it passed no problems.

So, a trip to the mechanic may be in order. New cats are not cheap (all things being relative).

Shortly after that, we decided to sell it and buy a new 2021 TRD ORP. Awesome decision for us in our situation.

ETA: Hope you get it fixed quickly and cheaply.

My mileage is at 191,831. Ok I’ll try replacing both and see what happens. Maybe that helps the monitor get ready. Hopefully I can avoid new cats. Thanks for the help!

Edit: Read your reply wrong the first time.

Do you have the right sensors in? Serious question? There’s even a video on YouTube discussing this exact problem on another Toyota vehicle. If you do not have exactly the correct sensor on that vehicle it will not go into drive cycle mode.

California 4runners do not have the same sensors as everywhere else. You absolutely must verify you have the right sensor. In fact, I would argue get a Toyota sensor for the wide band front, and the proper rear 02.

Toyota AFR / 02 Sensor Problems - YouTube

Sometimes the readiness monitors clears for me in 2 days. Sometimes it takes 500 miles. If you are in a tight pinch and literally have no time left... perhaps find a smog shop that will pass with one readiness monitor not complete? Some states will still let you smog it with one not ready, not really sure if CA is one of them as I always wait for all of them to show as passed.

Assuming everything else is set, you should be ok with one monitor not set. On pre 2003(?) model year vehicles, you have to do tailpipe emissions, so not all monitors need to be set. I believe my evap was not ready last time I passed on my '99.

-Charlie

Pretty sure it's no tailpipe sniff post-2000. All data is collected onboard by sensors.