P0420 Connundrum PLEASE HELP

I'm in CA and I was supposed to get my bi-annual smog in Dec. of 2018. The P0420 code has prevented me from doing so. I have a 1998 4wd manual with 380,500 miles.

-Has passed all previous smogs since I purchased it in 2010 (2010, 2011, 2012, 2014, 2016)
I got it smogged in March 2019 after turning the CEL off praying that somehow it might pass. PASS on emissions, FAIL on functional check (due to CEL)

-New CAT in 2012 (roughly 130,000 miles ago)
Old CAT was stolen
No rotten egg smell, isn't excessively hot, no loss of power

-Replaced o2 sensors (twice)
Haven't replaced them with Denso but could this really be my problem?

-Repaired exhaust leaks

My exhaust (tailpipe) broke off at the back of the muffler. Would that be an issue even though it's after the cat and sensors?

Any advice is much appreciated! I love my truck and it runs great despite it's age and mileage, I would like to keep it on the road!

 

I have a 2003 Tacoma. Broke down and replaced the O2 sensor with the $$ Denso one. When it comes to smog sensors, OEM parts are best. Also, check the wiring (a longshot, but possible). Loose or corroded connectors or damaged wire can give the computer faulty info.

https://www.autocodes.com/p0420_toyota.html

 

Thanks 4x! I've had conflicting advice on whether or not to go with the Denso's but I think I'm going to buy the sensors now. It's better than taking it to a mechanic and spending hundreds of dollars just to have them replace something I can already do myself!

 

Anyone says that OEM parts are not as good, or your wasting your $$, they don't know TOYOTAs. Mile for mile, TOYOTA parts are ALWAYS cheaper, hands down.

 

Denso manufactures OEM parts for Toyota (like Delco does for GM and Autolite does for Ford). You can get the “Toyota” part from the dealer for $$$, or get the Denso part from a “real” auto parts store (that supplies professional auto repair shops, not DIY places like Autozone or O’Reilly, etc) for $$.

 

Correct. I should have been clearer. I know TOYOTA doesn't make their parts. Just like the AISIN water pumps are the same as what TOYOTA uses, etc.