P0420 code won’t go away

If you installed an aftermarket CAT, most likely that is the problem. They vary in quality and some are not very good

 

What did you replace them with?

 

Try looking at the upper O2 sensor it seems that people just leave this alone I'm not sure why from the way I would think it works the two sensors see values and have to rectify the values in between them to not code or something like that everybody changes the downstream because it gets cut stolen removed it's stealing of the cat etc but the upper one is just sitting there for 25 years I get this same code for the last 2 years now and a gen 2 with the stock cat with no flow problems that I can tell in other words when the engine on it feels very much like a 2Z Corolla engine coming out the tailpipe basically at the same RPM that's all I have to go on and I get this cat below threshold it's been permanently on and another 05 that I put on the $100 cat same thing but it'll go off and stay off for weeks at a time couple of days at a time then all of a sudden come back on I want to say the downstream and the upstream O2 sensors are original they were unscrewed when the cat was stolen the lower one. And that's about it that car no longer runs but my '09 with the factory cat has had this code for 2 years I'm going to try changing both of the sensors not just one I don't think the denso brand matters I think maybe they both need to be kind of new to be close so they can sense the two ranges or something I don't know or care I would like to code to go away but it doesn't seem to be affecting any kind of gas mileage so who cares really