Catalytic converter

Stick a good scan tool on it and watch both of the O2 sensors. Look at the freeze frame data that goes with the code as well. You want to look at the engine rpm, fuel trims and O2 sensor data. Lean or rich conditions can trigger a P0420 code. A lazy O2 sensor can trigger it as well. Those items may not trigger a different code though.
A lot of people see that code and think "swap the catalytic converter", sometimes they are right, many times they are wrong.

Just to add my take on this one what I would be looking for is with the engine at operating temp and the engine rpms at about 2000 rpms watch the upstream and down stream o2 sensors see if the two sensors are reading about equal. That would show the cat isn't changing much

Hmm, my reply vanished.
SaturnTech9 has it correct. If both sensors are tracking each other the cat is bad.

Lean condition would show up as higher fuel trims as it tries to cover for something like a vacuum leak. The numbers will go down as you rev the engine because the leak becomes less of a factor as air movement increases.
Rich condition would show the opposite, it will try to remove fuel for something like a leaking injector or high EVAP purge.

Cat temp is a reasonable indicator but it can be hard to know how much of the cat is actually working. You could have a cat that is 1/2 plugged and the other half is running hot because of the increased load.

Your reply is still there Steve I just saw it.I had a f150 recently with the cat code I checked it that way and they were about equal. So I knew both o2 sensors were working and the cat was bad.

Is the car federal emissions or California emissions?

Also rock auto dot com has some good deals on cats that's where I always try to get mine.I have a 5 percent discount code for them if you need it.

Look on the sticker under the hood. It will tell you which emissions standard your car was built to meet. For instance I'm in NY, BUT every car sold in NY has to meet the California emissions standard. So if your car was originally sold new in NY you may have a CA emissions package.

As to the two different convertors, Check your car as it should have come from the factory with both of those on it. The front one is the primary cat, and mounts on the engine the one under the car is the secondary cat. The front one is monitored by the PCM, the rear isn't.

Imagee 1 shows the under-car piping. Note the single O2 sensor in front of the CAT. That is B1 S2. B1 S1 is in front of the catifold. See second image.

It says that code is the exhaust manifold cat there cheaper on rock auto dot com I also have a 5 percent discount code for them if you need it.

5970852653612978 put that in the how did you hewar about us box when you check out