A catalytic converter is one of the most expensive single parts on the car, and P0420 is the code that most often leads people to buy one. It is also a code with three quite different causes, only one of which is the converter.
Worth being careful with, then.
What the code actually says
P0420 does not mean the converter has failed. It means the computer compared the oxygen sensor before the converter with the one after it, and the difference between them was smaller than it should be.
A healthy converter stores and releases oxygen, which smooths out the rapid swings coming from the front sensor. So the front trace oscillates and the rear trace stays comparatively flat. When the rear starts mirroring the front, that smoothing has stopped.
Three things produce that result. The converter has stopped working. The rear sensor has gone lazy and is no longer reporting accurately. Or air is getting into the exhaust through a leak and reaching the sensor.
Check the other codes first
This is the step that saves the most money and it costs nothing.
If a misfire code is stored alongside P0420, the misfire is the job. Raw fuel passing into the converter and burning there overheats it, and it will destroy a converter in a surprisingly short time. Fit a new converter without fixing the misfire and you will be buying another one.
The same applies to a lean or rich condition. P0171 alongside P0420 means the mixture has been wrong for a long time, and the converter has been paying for it.
There is a general principle here worth stating plainly: converters almost never fail on their own. They are usually the last casualty of something else that went unrepaired. Finding what that was is not optional, it is the difference between one repair and two.
Check for exhaust leaks
Any leak between the engine and the rear oxygen sensor lets fresh air into the exhaust stream. Extra oxygen reaching the rear sensor makes a working converter look like a failed one.
Look at the manifold for cracks, at the flange gaskets, and at the flexible section if there is one. Do it cold and again warm, because some cracks close up as the metal expands.
This is a cheap repair that fixes the code outright, and it is missed regularly.
Compare the sensors
With the engine fully warm, hold a steady 2000 rpm and watch both sensors on that bank.
The front sensor should be swinging quickly and continuously. If it is not, the problem is the front sensor and the converter test is not meaningful anyway.
The rear sensor should be comparatively flat. If it is swinging in step with the front, the converter has stopped buffering.
If the rear sensor barely moves at all, or moves very slowly and lazily, suspect the sensor rather than the converter. A worn out rear sensor is a fraction of the cost.
On a V6 or V8 with no code on the other bank, you have a free reference. Compare the two rear sensor traces on the same engine. That comparison is far more conclusive than looking at one trace and deciding whether it looks flat enough.
On aftermarket converters
If you do conclude the converter is finished, be aware that cheap aftermarket units frequently set the same code again within months. They contain less of the precious metal that does the work, so they buffer less oxygen, and on some vehicles the computer is sensitive enough to notice from new.
That is not an argument that every part has to come from the dealer, but it is worth knowing before choosing the cheapest listing, because the labour is the same either way.
And on the products that promise to clear it
Fuel additives sold to fix P0420 will not repair a converter that has lost its coating or melted internally. Where they occasionally appear to work is on a converter fouled by oil or fuel deposits, which is a narrow case.
If the converter is genuinely failed, nothing poured into the tank brings it back.