Home TroubleshootingExhaust P0420 Code – Everything You Need To Know About This Code

P0420 Code – Everything You Need To Know About This Code

by Kelvin Yates

So, you’ve finally decided to get rid of that annoying check engine light. But when you scanned the car for error codes, did you get a P0420 code? What does this code mean and what you should do next? First, you shouldn’t worry because we are going to dissect this problem in detail. Here’s everything you should know when it comes to the P0420 code.

Getting a check engine light and error codes is unfortunate. But it is what it is, and you should try to solve the problem as soon as possible. This will help us to return your vehicle in proper working order. Since you got the error code, you can now try to find a solution.

The P0420 code is an error code that usually turns on when you have some issues with your exhaust system. This code is telling you that something isn’t right and you need to take care of this issue. But what can you do to get this thing done quickly and effectively? Well, we will explain everything.

In this article, we are going to learn everything you need to know when it comes to the P0420 code. We are going to explain what this code means. Then, we are going to cover the symptoms that you are going to experience when you are having this code. Then we are going to cover the possible culprits for this problem and then we are going to discuss the solution. And lastly, we are going to cover the cost to fix this error code. So, let’s dive into it.

What Is The P0420 Code?

The P0420 code is an indicator that there is something wrong with your exhaust system. More precisely, your catalytic converter doesn’t work as it should. The catalytic converter is a component whose role is destroying the bad particles and not letting them leave into the atmosphere.

The P0420 error is telling you that there is something wrong with the bank 1 converter. V6 and V8 engines have two catalytic converters. One per each engine bank. So, the bank 1 converter is usually on the side that has the number 1 cylinder.

This is going to be of big help to understand the issue and to know where to look and fix the problem.

Why Does This Error Appear?

This error appears because of the O2 sensors. One of the sensors is located before the catalytic converter and one is located after the catalytic converter. They are measuring the level of oxygen inside of the exhaust. Since the catalytic converter is clogged and doesn’t let the gasses pass through freely, it makes the O2 sensors show bad readings.

These readings that the O2 sensors showing are red alerts to the PCM (powertrain control module). When the PCM gets enough of these bad signals, it shows a check engine light on the dash. This check engine light is a sign that you cannot ignore and you need to further investigate the issue.

The only way to investigate the problem is by getting an OBD2 scanner and scan the car for error codes. A simple OBD2 scanner is fairly cheap and easy to find online or at some parts stores.

Then, you have to plug the scanner into the port and let it scan your car. After the scan, you will be shown some error codes if there are any. And if you get a P0420 code, it means that you need to act quickly and check your catalytic converter.

But besides the code, some other symptoms appear when the catalytic converter is broken and we are going to discuss these next. That’s because if you have a bad catalytic converter, you are going to see some other unusual things besides the check engine light.

What Are The Symptoms When This P0420 Code Appears?

When the catalytic converter is broken, there are several symptoms that appear and can basically frustrate you in some way. They are a clear indicator that the catalytic converter is broken and you need to check its condition before you make any steps. But what are the symptoms of a broken catalytic converter? Well, we are about to find out.

Poor Engine Operation

If the catalytic converter is broken, you will get this error on the dashboard. You can also expect that your engine will work poorly and will not deliver a lot of performance.

This is the case because the catalytic converter is clogged and doesn’t allow the gasses to pass through, which creates backpressure. In engineering, the term backpressure means slowing down. The engine basically slows down, since it cannot manage to expel all the gasses from the exhaust. It doesn’t matter if you step your foot on the gas. The engine will not drive any faster and you have to try and fix this problem.

This can be a frustrating thing to drive a vehicle that has trouble keeping the engine running, even if everything is ok with the engine.

The only way to deal with this issue is by checking the catalytic converter and seeing its condition, and then move on from there. There are ways to save a catalytic converter and we are going to discuss them at the end of the article in detail. But if the catalytic converter is too far gone, you are probably going to have to replace the catalytic converter with a new one.

Check Engine Light Illuminated

When a catalytic converter goes bad, the check engine light appears. This is the case because the sensors that are measuring the exhaust gasses before and after the converter are not getting the proper readings. This means that either one of the sensors is broken or a catalytic converter is bad.

That’s why you need to investigate this issue further and learn more about the check engine light. In doing we, we are getting an OBD2 scanner and scanning the car for codes. This will tell you a lot more about the condition of your car and the possible faulty components.

P0420 Code

For example, it’s going to tell you if the sensor is bad or if you have an issue with the catalytic converter. If you get a P0420 code, the most probable cause is a bad catalytic converter on bank 1. From here, you have to move on and look into the catalytic converter and its condition.

The catalytic converter has to be removed and thoroughly checked and inspected for possible clogging. On top of that, the overall condition needs to be checked if it can be cleaned. If not, you are going to need to buy a new catalytic converter.

Failed Emissions Test

A failed emission test is also one of the symptoms of a bad catalytic converter. Every time you go to register your vehicle, the technicians will measure the exhaust gasses. And if your car pollutes more than it should, it means that there is something wrong with your exhaust system. Or, the engine is running too rich.

In this case, if your results are bad and your engine pollutes more, they will simply fail your test. Then, they’ll tell you to fix your car and bring it back to get it registered.

This can be a frustrating thing, but that’s how the rules are. You have to comply if you want your car to be registered and driven legally.

If you fail your test, you need to look for a solution immediately and see if you can find some old catalytic converters on the second-hand market. And if there are any available, excellent. If there aren’t any second-hand catalytic converters for your vehicle, then you will need to replace your catalytic converter with a new one.

So, how much for a catalytic converter? This can be pretty pricey, honestly. A new catalytic converter can cost more than $1,500.

Rotten Egg Smell

The rotten egg smell coming from your exhaust is another dead giveaway that your engine is running too rich. Or, the catalytic converter is broken.

The rotten egg smell usually starts to appear because there is sulfur in the fuel. If there is too much fuel in the system, or the catalytic converter is broken, the sulfur will not be burnt and will come off from the exhaust system at the back.

This will result in a rotten egg smell. This smell can be really annoying and you are not going to enjoy driving like this. So, you need to fix this pretty quickly. And why?

Well, the number one reason is that you will not be able to pass your emissions test and register your vehicle. And the second reason is that your car is going to pollute a lot. Maybe ten times more than it should. And if the police spot that your car is smoking and polluting, you might be stopped and fined and then sent to pass emissions again.

That’s why it is recommended to fix this problem as soon as possible and not let the problem become bigger and also avoid getting fined.

What Are The Possible Culprits Of A P0420 Code?

If you get the P0420 code it doesn’t mean that your catalytic converter is broken and you have to replace it at once. This problem should be investigated further because several issues can prompt this code on the computer and ruin your day. In this chapter, we are going to cover all of the possibilities that are on the table, and then you need to move from there in understanding the issue and fixing it. And that’s something that we are going to cover next.

Broken Catalytic Converter

The number one reason why the P0420 code appears is because of a bad catalytic converter. The catalytic converter simply does not do its work properly and the PCM is recording a number of errors from the O2 sensors that are measuring the exhaust gases and sees that there is something wrong.

Then the PCM decides that it’s time to throw an error on your dashboard and make you notice the problem. You can’t know right away what the issue is and you need to investigate the problem in depth. And that is by scanning your car with an OBD2 scanner.

P0420 Code

If you scan your car with an OBD2 scanner and you get the P0420 error, you can be sure that there is some kind of problem with the catalytic converter on bank 1.

Then, you can start the repair process and this will require removing the removal of the catalytic converter and giving it a deep cleaning. Or if it’s too broken, you will have to replace it with a new one.

Later in this article, we are going to discuss how you can clean your catalytic converter without removing it. There are some tricks in the book that will do this job without all this work.

Bad O2 Sensor

Another reason why you may get a P0420 code is because of a bad O2 sensor. This happens less often. But sometimes, O2 sensors can mess up and cause all sorts of issues. That’s even if they continue working and you don’t get errors on them.

It’s rare, but it can occur, thus allowing the sensor to give false alerts to the PCM and to get a check engine light. This will be proven when you remove the catalytic converter and inspect it for damage. If the catalytic converter is ok. You can start looking at the sensors and check their condition. You can also test them with a blow torch and see if they are making any voltage when heated up with a torch.

If they are not making anything. Then probably the sensor is broken and you have to replace it. O2 sensors are not the cheapest, but they are also not that expensive. You are going to pay around $100 for a new one. But always look to get one that is OEM spec and not get some of the cheap knock-offs.

Holes In The Exhaust

Holes in the exhaust can also cause the P0420 code to appear. Also, less often but they can happen. These types of problems are called exhaust leaks.

If your exhaust has leaks you will notice a hissing sound. Or, a strange loud sound if there is a big hole in the exhaust.

These small holes can also appear in the catalytic converter and cause the error P0420 to appear and really annoy you. That’s why if you think that the catalytic converter and all the sensors are good, you should see if there are any holes and leaks in the exhaust system.

If there are holes you will have to patch them. Patching holes can be done in multiple ways. Like welding, covering them with a leak preventer, and attaching clamps that will hold the leaks.

All these methods are cheap and easy to do. But if you want a permanent fix, you will have to replace the damaged catalytic converter with a new one. Because the hole can again open and cause issues.

How To Fix the P0420 Code?

There are some ways how you can solve this problem and fix the error P0420. We are going to cover all of them in this chapter. Some methods involve removing the catalytic converter and there are ways that you can get away from this problem by following some simple steps. So, let’s jump into it.

Highway Run

A good thrashing on the highway is probably going to help you out immensely if your catalytic converter is bad.

I’m saying this because the catalytic converter gets clogged up because the car is driven through town a lot. And this type of driving does not allow the catalytic converter to come to operating temperature and heat up. Heating the catalytic converter is going to have a positive effect and all the debris that is inside of the converter will basically burn away.

A good run of 15 to 20 minutes on higher RPM is going to help a lot. But only if the catalytic converter is in the early stages. If it’s too far gone you will need to replace it.


CataClean is another solution that is popular on the market. This solution is poured into the fuel tank and then you need to drive the car on the highway for half an hour or so for the catalytic converter to get up to temperature. And once the catalytic converter gets up to temperature, this solution is going to help destroy all the debris and gunk that clogged up your catalytic converter.

P0420 Code

People report that it helps a lot, and if you want, you can give it a try. It’s not that expensive after all. It’s far cheaper than removing the catalytic converter. We are going to cover that one next.

Removal And Cleaning

If nothing helps to clean your catalytic converter. Removing the catalytic converter might be a good idea. For this job, you need to jack your car in the air and get underneath.

You should also note that bolts can be rusty and can be stuck. For this, WD-40 would be useful. Then after removing the catalytic converter, place the converter in a bowl in which has lacquer thinner.

Leave it overnight and check the condition in the morning. The debris should be a lot looser. Get a hose with high pressure and give it a good rinse from both sides. A high-pressure washer is also going to do the trick.

Repeat the process a few times and after this, the catalytic converter should be clean and all of the debris removed. If some of the debris is left inside you can try again running the car on the highway for everything to be removed and the catalytic converter cleaned up.

If nothing helps, unfortunately, you are going to need to replace the catalytic converter. And we are going to cover more about the costs in the next chapter.

Cost To Fix This P0420 Code Error

The cost to fix a catalytic converter and the P0420 code is expensive in the worst-case scenario. And that is to remove the catalytic converter and replacing it with a new unit.

Catalytic converters tend to be expensive because they are built from many precious metals that cost a lot of money. That’s why the end price of a new unit can be above $1,500 depending on the vehicle that you have.

Some catalytic converters can cost even $3,000 for some makes and models. That’s why you should try cleaning the converter first if you don’t want to have any more issues with it.

There is also another way, and that is catalytic converter delete. The catalytic converter delete is a good idea if you live in some places where laws are looser and do not require all the environmental equipment on the car.

If you live in a place like this you can just cut off the catalytic converters and replacing them with pipes. This will save you a lot of money and you will get a better sound from your car. Also, the horsepower numbers will be increased since the catalytic converter is no longer there to block the exhaust gasses. That’s why this option is worth considering if you have a problem like this with your car.

Facts about Code P0420: Definition, severity, and repair recommendations

  • Code P0420 means “Catalyst system efficiency below threshold (Bank 1).”
  • Driving long-term with this code can cause engine and exhaust system damage.
  • The most common cause of P0420 is a bad catalytic converter, but it can also be caused by other issues, such as a faulty oxygen sensor, a rich or lean running condition, or misfires.
  • P0420 means the catalytic converter is not efficiently removing pollutants from the exhaust, which can lead to failing an emissions test.
  • Misfires or incorrect air-to-fuel ratio could be the causes of a failed catalytic converter.
  • A bad catalytic converter isn’t a hazard to the driver but it can overheat and destroy engine components.
  • Fixing P0420 can cost anywhere between $0 and $2400 depending on what the cause is.
  • Fixing P0420 at home depends on the cause of the issue, some repairs can be done by the car owner, such as replacing a broken or damaged oxygen sensor, patching an exhaust leak, or replacing wiring or connectors, but some other repairs should be done by a mechanic, such as replacing a worn catalytic converter.
  • Before fixing P0420, car owners should correctly diagnose the cause of the code to avoid spending money on unnecessary repairs.
  • To diagnose P0420, it’s important to make sure there are no other codes present, to check if the code was random, to check the exhaust system for leaks, and to check the downstream O2 sensor to see if the catalytic converter needs to be replaced.

P0420 Code – Conclusion

In this article, we have covered a lot when it comes to the P0420 code. We learned what does this code represents in the first place, and that is a problem with the catalytic converter on bank 1.

Then we learned all of the symptoms that are connected with a bad catalytic converter for you to have a better idea of what is causing your problem.

Then we have covered the possible culprits for this problem and that is the broken catalytic converter. In this case, the catalytic converter needs a good cleaning or replacing.

We also covered the ways you can clean your catalytic converter and the cost to replace a catalytic converter. Also, if you live in a country that doesn’t have strong emission regulations you can just cut off the catalytic converters and replace them with straight pipes.

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