P0420 OBD-II Trouble Code – Meaning, Causes and Fixes

If you happen to see the engine light turned on your car’s dashboard with a P0420 code, you should immediately pay a visit to your mechanic. Simply speaking, a P0420 code is directly correlated to the engine which is underperforming in some way.

However, it is not easy to determine if your car is underperforming or just having an electronic issue. Sometimes, a ‘’check engine’’ light on your dashboard may be false alarm, but if it isn’t, this can leave some serious damage to your car engine.

Also, you probably won’t even know that you have P0420 engine issue until you visit the mechanic. In that light, let’s see everything that you need to know about the P0420 code. In order to do the right thing when it comes to your car’s performance.

What Does P0420 Code Mean?

There isn’t a nice way to say this, but P0420 code implicates that you have a serious car issue and that your engine is in trouble. There are synonyms for P0420 code such as ‘’OBDII code’’, ‘’diagnostic trouble code’’, and ‘’check engine code’’. All in all, they all tell you the same thing, that your engine is underperforming and it needs a professional help.

Anyhow, the P0420 really stands for ‘’Catalyst System Efficiency Below Threshold’’. That being said, if you see the P0420 code, this usually means that your catalytic converter is not working at its maximum efficiency.

Simply speaking, this problem can mean two things. First, that you need to replace your catalytic converter. And second, that there might be a problem with an O2 sensors. Anyhow, one thing is certain, you will probably need to replace the catalytic converter.

Namely, it happens to a lot of people that they don’t actually notice whether their catalytic converter is underperforming or not.

However, the environment certainly will notice that. This is because the catalytic converter is helping your car to reduce the car emissions. With the ability to convert the carbon monoxide into the carbon dioxide and water, the catalytic converter helps saving the environment and planet earth.

In other words, your car will definitely not pass the vehicle emissions test or smog check. So, you will not be able to drive that car for sure.

What Causes P0420 Code?

Now, let’s see some of the main causes when it comes to P0420 code that your car is showing you.

1) O2 Faulty Readings

We already said that there might be a chance that your O2 sensors aren’t working properly. In that light, all you have to do is to change these sensors and you will be all good to do.

2) Failure of the catalytic converter

This cause of the P0420 code is definitely the worst. This means that you will have to take an expensive trip to the mechanics. If there is a problem with a catalytic converter, you will have to replace it right away in order to save your engine from further malfunction.

3) Exhaust system damage

Sometimes, the P0420 code seems for damaged exhaust system. Firstly, you should check the exhaust system for any leaks or damages and get it to the mechanics to fix it.

4) Oil contamination

Now, the catalytic converter requires oil to work properly. If the oil is contaminated, the engine light will turn on. This means that your catalytic converter is not damaged heavily and all you have to do is to change to oil.

5) A fuel injector leaks

If there are leaks in the fuel injection system, your engine light will turn on and you will have the P0420 code problem. This is also a serious problem that only a mechanic can fix.

6) Wrong fuel use

If you use a wrong fuel for your vehicle, the engine will suffer and therefore the catalytic converter will be damaged as well.

Is it necessary to replace catalytic converter?

Replace catalytic converter

Via: https://www.cars.com/

As most of you now wonder, yes, it is definitely necessary to replace your catalytic converter as soon as possible. This way, you will no longer be a treat to an environment and you can actually pass the smog test. Also, get your vehicle registered when the time comes.

Now, sometimes it doesn’t have to mean that you need to replace your catalytic converter if there is a problem with the O2 sensors. Namely, the main purpose of O2 sensors is to monitor the fuel and air mix in the exhaust which will allow your engine to perform normally.

If the sensors give the wrong information about the air mixture, your engine will adjust to it and it will definitely underperform.

However, if there is a problem with the catalytic converter, you should definitely get it replaced with the new. It will save yourself a lot of trouble.

How to Fix P0420 Code

Simply speaking, there isn’t an easy way to fix this kind of problem. Namely, it depends on the cause of the problem to know if you can fix it or not. Usually, people go to mechanics and let them take care of that. But, let’s see can you expect and how to fix this problem.

  • Replacement of the O2 sensors – If there is a problem with the O2 sensors.
  • Replacement of the Catalytic converter – If the catalytic converter is damaged.
  • Replacement of the exhaust pipe or repairing the pipe leaks – If the exhaust system is damaged.
  • Replacement of the fuel injector – If the fuel injector is leaking.
  • Diagnose any other problem that needs repairing.

Diagnostic Tips

There are lots of ways to question the possibility of P0420 code problem. But the best one is to get yourself a diagnostic scanner that will show you the problem. However, there are some other ways to determine if you have this kind of problem.

  • Vehicle will lose power after the engine warms up.
  • Speed of the car will not exceed 30-40 mph.
  • Uncomfortable smell of rotten eggs.
  • Problems with the acceleration.


All in all, the P0420 code is definitely a serious and quite important thing for your car. Your car will slowly lose its power if you decide to ignore this problem. Also, it can causes some serious problems to your engine.

In addition, the best way to prevent these things from happening is to take care of your car properly. And check for the symptoms every now and then. We surely hope that this helped you and your car to fix a problem.

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