Trouble Code PO420

I'm having a case of deja vu from my time with my 1999 4Runner, which had a very pesky issue with catalytic-related trouble codes. I never solved the underlying problem, and eventually sold the beast to get my '04 Tacoma, a 4-banger. It only has about 50K on it (I bought it earlier this year with 44K on it), but i'm getting trouble code PO420 ("catalytic efficiency below threshold"). before i make all the same mistakes I made with my 4Runner, i was hoping someone here may have tackled this issue with their 4-cylinder Taco, and can provide insight/advice.
THANKS!!

 

and they said the basic warranty (3yr/36K) is up. asked about emissions, and said it was not different...

 

nope, not in NY. BUT, I just saw an article that says the federal warranty is supposed to cover the following parts:

Catalytic Converter
Catalytic Converter Shield
Transmission Control Module
NGV Module
Powertrain Control Module (PCM)

so, i guess it would be a matter of having the dealer do the diagnosis, which will get pricey in its own right, and then if it is one of those parts, i "should" be covered.... it's tempting to try and tackle this myself...

another question:
how many o2 sensors does the 4 cyl taco have, one? two? my 4runner had two: an upstream and downstream... which made finding the problem on that truck that much more of a pain...
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emissions is govt mandated i thought 80 k miles look into it my friend got her cats replaced in her mitsubishi and they covered it and even replaced the o2.


also when i run supreme my v6 2nd gen will throw this code but with regular it will go away after a few days of running straight regular 87. Its weird