Can I do incremental replacements to fix P0420? What order?

You could try a a cat soak recipe. There's plenty of them online. That might clean it out. However the aftermarket replacement cats aren't very expensive. Here's the one we recommend if you don't live in California or New Jersey: https://www.ebay.com/itm/193690871378

 

You're so far replacing my cat in my generation 2 with the popular aftermarket still yields the code I have not played with the sensor as it's factory not cut It looks good even when looking at it unscrewed I haven't tried spacing it or anything I will replace it soon and see what happens just for the heck of it but the code 0420 is always there the light is always on if I reset it it might go away for possibly a day and then it's back on it's been that way the whole two and some change years that I've owned the car. Doesn't seem to affect gas mileage I'm sitting at 50.1 right now etc etc I can't imagine it's just the cat even a new cheap cat should work enough to generate the voltages you need at the O2 sensor up or down

 

If you have any exhaust leaks, you need to remedy that before doing anything else.

 

Check the 2 inch black plugs under spare tyre .fumes will enter here in my 09 right smartly with just one of the 2 moved or un capped. I know it immediately. Put zip tape over plug it's now permanently closed off.

 

Check the donut right at the exhaust manifold with the spring bolts this is a common leak point on general motors using this exact same type of connection usually if the car bounces or flies down a dirt road as that connection flexes it can leak but usually when the road levels off and everything's back to smooth the leaking will stop unless you hear it it's probably not happening The joint from the cat back can always be suspect sometimes double gaskets are needed there or extra thick exhaust gasket from the muffler shop place there draw those bolts up tight on that gasket before you start slipping all the posts into the rubber mounts. In my Corollas and whatnot I've always removed one of the rubber plugs in the spare tire well so when water gets in there it has a place to run out of the car not puddle in a Corolla you're exhaust smell won't run up those holes in the Prius the exhaust pipe sits under the car almost hidden and fumes when the car is sitting will tend to run right up that hole in the trunk that you uncovered or that I uncovered in the Prius and go right in the car I had to put the plug back in the Prius so now I have to constantly look in the spare tire well just to make sure in the winter time especially it's not getting water sitting in it I don't carry a spare too much extra weight.

 

There are a number of threads in the Gen2 forums concerning P0420. Search and read.

Short form: NO exhaust leaks from the engine cylinder head back to 2 feet past the downstream O2. No engine misfires. Air/Fuel and O2 sensors have been tested and proven to function correctly. Engine (total) fuel trim numbers are 0 +/- 10%.

If all that's "good", then it needs a cat.

In order to determine whether everything is in working order is going to require some self-education, and purchasing a more capable scantool.

If money is really a concern, then there's the question, do you need to deal with this right now?

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Most any exhaust shop will jack up your car and tell you if you have any leaks and stuff they can spot weld or whatever If you have any loose bolts worn out springs at the spring joint whatever shouldn't be very big of a deal I've never had a gin too so far that's had an exhaust leak I've had the exhaust pipe too far under the car and it comes up into the spare tire well but no leaks on any of the joints or anything all near 300,000 when this was checked I was getting the PO420 not in the lip mode or anything get an excellent gas mileage matter of fact my '09 still has it and I have an aftermarket what is it 48 state cat or you know whatever it is the one that can't go to California but it's made by one of the biggest manufacturers that makes converters and whatnot so I don't think that's the problem

 

Used up cat still has the same value for scrap. I got 800 for mine here in NH. After market part was 130 and I am no longer a target for theft. I replaced both o2 sensors at the same time 1st and immediately got the code for the cat. OK for the bad news I just started getting the 0420 code again after 9 months I think I have fouled the cat from the 2 qts of oil burning between changes and have ordered another. 245k on 08 gen 2 averaging 45mpg the car owes me nothing.