What can I do about this?

It means you need a Bank 1 converter. it's not an 02 sensor.

read this on how to fix a P0420

P0420 - Catalyst system efficiency below threshold.

Probably just a bad O² sensor.

Lookit: https://www.obd-codes.com/p0420

You see a mechanic with a dealer level scanner. He will check the front and rear O2 sensor readings. He will do a check on the front O2 sensor to see if it is working properly. He will check the engine fuel trims to check if the engine is running right (not running rich or lean). He will check for an exhaust leak. The results of the tests will tell the mechanic if the cat converter is bad or there are other issues with your vehicle.

It will not make the car stop working but the car will not pass an emissions test that way..

P0420 (catalyst system efficiency below threshold [bank 1]) means the oxygen sensor past the catalytic converter is reporting high and fluctuating levels of free oxygen where the level should be relatively low. The assumption is the catalytic converter is not working right, and that is half true. It is not working right but that is often because it is receiving indigestible exhaust.

I doubt the condition is ever because of a bad downstream O2 sensor, but the upstream sensor - the one under the hood - can contribute to the error code if it gets very old. The life expectancy of O2 sensors is about 100K miles, after that they get sluggish and do not help control the mixture well. Dirty fuel injectors have the same effect: poor mixture control. As Jay P points out, a common cause of P0420 codes is an exhaust leak ahead of the converter. Finally, it is possible the converter itself is bad, although in none of the dozen or so P0420s I have dealt with did the converter have to be replaced.

Because it is not a serious operational issue I recommend you take it in order of the cheapest to the more expensive.

1) If you have been using whatever gas is cheapest, fill the tank with a top tier brand (see the first source) or add a bottle of Techron to a tank of random gasoline. My daughter filled up at the local Love's regularly, and when her Sienna developed a P0420 I put a tank of Mobil in it. Three days later the light cleared and stayed that way for a couple months... she went back to Love's as soon as the Mobil was done. Don't let anybody tell you gas is all the same! Plenty of the top tier brands are bargain brands as well, like Conoco and Quik Trip.

2) If you go through the tank of cleaner gas and the light is still on, take the car to a muffler shop and have them look for and repair any exhaust leaks. They are quick and good at this; same test. If the light does not clear in a week it is time to take it to a mechanic when you can afford it.

3) If i were doing it I would replace the upstream O2 sensor. Not a great suspect but they age and reduce throttle response as well as make the P0420 code more likely.They are in the $100 range - sometimes into the $200 range - so I don't recommend that lightly. With labor it gets into the long shot range, so this part is more informative than recommendation.

4) Replace the converter. Prices vary widely, depending on the vehicle and where you live (like California or New York, where the requirements are very strict/expensive.) I do not like this option. Not only is it expensive but it has a dark path ahead. I can almost guarantee it will turn off the check engine light and it will stay off for about two years. That is great for mechanics, as long as you don't bring it back when the light comes back. The new converter works better enough to get the "measured" efficiency above the threshold until the underlying condition gets worse and the new wears off the converter. Replacing it again buys a few more months. Occasionally the converter was really bad and the new one lasts for years... but, as I say, in the dozen or so I have done none needed a new converter.

Overwhelmed? Remember - it will never leave you stranded unless something in the mixture control is behind it... and it is rare for anything that bad to announce itself with a P0420.

By the way - it would help to know make, model, and year.

Sometimes a dirty sensor in the exhaust system can give a false reading. It's worth taking the car for a fast run which will burn off any carbon fouling. Try that as it costs nothing. But as said it may be a faulty sensor or, much worse, a failing catalytic converter. If you are losing power the cat is suspect.

It's mostly an emissions issue. The car's computer thinks there might be something wrong with the catalytic converter, which is there to burn the exhaust gases more completely on the way to the tailpipe. There are oxygen sensors ahead of and behind the catalytic converter to monitor how well it's working. Bad results can occur if:

- There is an exhaust leak

- An O2 sensor is failing and giving bad readings

- Low quality exhaust from the engine (bad timing, unburnt fuel, oil contamination, etc)

- The cat converter is actually failing

Start with the simplest/cheapest causes and work your way upward, but don't just swap parts without diagnosing the real problem.

Keep in mind that OBD codes do not necessarily tell you what part is faulty. It just tells you where the fault code is coming from.

An O2 sensor code could indeed be a faulty sensor but it could be an issue with the wiring. I know of one person that had a sensor code and it turned out to be a pinhole in the exhaust pipe just ahead of the catalyst. So what was happening was fresh air was getting in behind the front sensor and being read by the rear sensor. It drove him nuts because he thought he was getting faulty replacement sensors.

The moral of the story is that you should still diagnose the issue, not just blindly replace sensors just because you're getting the code.