P0420 code with non CARB catalytic converter for CA Prius

Part of the CA smog inspection, they normally have to make sure the catalytic converter is CA compliant. So you would fail if they check properly, even without a check engine light. Of course in CA, not all smog techs do their jobs properly so you may get lucky if the check engine light goes out and you’re able to go to one of those techs

 

It could be both or either. Since you were never able to extinguish that error code, the wrong sensor could've been installed or it was miswired back together. You'll need to look at the data the ECU is seeing to determine what's bad - the sensor or CAT or both..

 

It's because the non-compliant non-factory cat cannot pass the factory flood test that the car does to the converter however often it does it so when it does that test and it can't pass this happens on the car it can't pass that flood test or whatever the test we're going to call it and then it sets that code generally apparently for this particular vehicle only no Corollas none of the other vehicles maybe the other hybrids possibly but the Prius uses a special converter I guess that has a lot of extra of the precious metals in it that will allow it to pass this flood test a normal converter just floods out overloads itself and can't pass the test you get the code this is the catalyst below threshold code It has nothing to do with O2 sensors the lack thereof or anything else it is just this one test that the car runs I am shocked somebody hasn't hacked the firmware and turn that off but be that as it may apparently it hasn't been done yet. What you can try and do with the setup that you have now is get a spacer for the downstream O2 sensor that screwed into the pipe right there near the cat or in the cat or whatever not the one up in the manifold the one down in the pipe they make this 90° swivel out of stainless steel that has a piece of catalyst material in the fitting You screw this where your O2 sensor was then you screw your O2 sensor into this fitting then you can rotate the fitting to where it sits nicely and lock it down with the nut supplied plug up to O2 sensor clear any codes that you have and drive and see if that spacer keeps the check engine light off It has worked for other people I have one sitting here I need to put on my gold work car it's had the 0420 code for about 4 years now I just haven't taken the time to put on this fitting they're sold all over eBay and Amazon for like $18 for a nice one.