New exhaust and still p0420

Your problem may just be the "cheap cat from Ebay". They don't always do what they're suppose to do.

 

It is possible but don't fret I have the same problem and I bought the cat from the big cat manufacturer that sells on eBay and they make a cat for the gen 2. And it looked and fit perfectly it's very similar looking at the brick to the original it's sealed up fine front and back it's welded in place by my regular shop and the code never went away in this setup The downstream O2 sensor looks like it's doing its job and the numbers look okay in tech. But other than that the '0420 was always on and always standing still getting 47.1 miles to the gallon and I don't have emissions in Orange county North Carolina so I'm golden All I can think to do is take the check engine light bulb out or LED out I checked the car so often with code readers and tech that I really don't need the light the triangle pretty much do the trick. It must be the test that the system runs on the cat at each startup to me that's a farce somebody should be able to break the software code and figure out how to make that not happen or not need to happen some firmware update for something but that looks to be the issue.

 

The test doesn't appear to run at startup. It will run at most once in a drive cycle, but only when the coolant is warmed above 75 ℃, the engine is running in closed loop, it's not idling and also not above 4000 rpm, the upstream and downstream sensors are both known to work, and both cat converters are in an operating temperature range.

I took those conditions from a Gen 3 manual, but I doubt much is different there from Gen 2.

And yes, "that looks to be the issue", in that if the car never tested cat function, you'd never see a P0420—same way halting COVID testing would have solved that problem.

 

I don't think my Corolla and Camry and non-hybrid models check the catalytic converter especially as described for this vehicle before. So I don't know I just know the cat that I bought for my Gen 2 was from a pretty big manufacturer of catalytic converters throughout the US supply chain for lots of places so I would think they would have a clue what's happening and all of that maybe not. Oh well I don't have emissions everything is spot on otherwise. So I guess just keep getting it I can justify over $1,000 cat. But I can keep driving

 

How many miles on the vehicle, and the old catalytic?

Which o2 sensor did you purchase as a replacement, was it a Denso/OEM, and did you get it from the dealership or online?

Why the downstream sensor and not the upstream, was it faulty or out of spec?

Was that the originial OEM cat, and did you save it? (it has quite a bit of value as is, and you can also attempt to clean it for reuse)

 

You may need to use techstream (dealership technician diagnostic software) to test and monitor the vehicle. There are other options as well.

Regarding the oil burn, have you thought about piston soaks, and or running special oil(s) to breakdown the carbon buildup?

Upstream A/F sensors doesn't last forever, and some may show signs of failure sooner than downstream oxygen sensors.

How many miles on the vehicle, and or the upstream A/F sensor?