Try cleaning first, but find the cause to prevent it again!
If the catalytic converter is dirty I can see how cleaning it would work. Try a gallon of lacquer thinner in a 10 to 12 gallon tank. P0420 comes with just 5% below correct efficiency threshold. If it was broken, it definitely would not work. (Melted or broken up.) Also, remember, if your catalytic converter went bad, be sure to look for any leaking fuel injectors, check timing and spark, as these problems make catalytic converters go bad. (To test fuel injectors requires some advanced equipment.)My point about trying to clean a catalytic converter, is that it just has some carbon build up on it keeping combustion gasses from getting the catalyzing metals, then cleaning it off will work. If enough of these metals are eroded away off of the ceramic honey comb, no amount of cleaning will make it work. If the catalytic converter doesn't have to be cut out of the car, then it may be practical to unbolt it and leave it in a thick concentration of dawn detergent over night, rinse it off, reinstall it and it may work again. This is a more aggressive and more effective way, but if your car is really old, my guess is that it just needs replacing.
Back to the analysis, has it really gone bad?
On a live data scan you can see the stoichiometric up and down from lean to rich on traditional O2 sensors. Seeing that on bank 1 sensor 1 or bank 2 sensor 1 - O2 sensors that are prior to the catalytic converter you're fine. If you see that same, or nearly the same wave form on sensor 2 for bank 1 or 2, then you should have a bad catalytic converter (saying this to cover V6/V8 cars with two engine banks, just apply the information to to a 4 cylinder or inline 4, (L4) engine without the bank 2 information. The sensor 2's on bank 1 or 2 are after the catalytic converter. Bank 1 catalytic converter below efficiency is a P0420, and a P0430 is a the same problem on bank 2.
I had an exhaust leak prior to my cat (a nickname for catalytic converters). When I replaced my cat I was lucky that I did indeed need it.The new cat almost stopped the wave form on the sensors behind the cat. You see, as outside air burps into your exhaust via the venturi effect (move gases down a pipe at high speeds will pull outside air in through a hole) it'll cool the exhaust gasses, and make your catalytic converter cooler for a moment, and burn less of the unburnt combustion gasses. A human can see the difference but the car figures exhaust leaks as a bad cat.The rear O2 sensors should be pretty steady. They can stay steady rich or lean and you've beat the P0420 and/or P0430.
They can go from lean to rich at times. My car's cat lives super lean, by my O2 sensors estimation. If I'm accelerating for a while onto the freeway or the exhaust isn't warmed up yet, then that sensor will go rich. Some rear O2 sensors will see rich all of the time. That's how usual catalytic converters/rear O2 sensor relationships are.
by AutoBravado
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