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How to fix P0420 code: “Catalyst System Efficiency Below Threshold.”

How to fix P0420 code

If you're searching for how to fix the P0420 code, your problem is most likely related to issues with the catalytic converter. Since the catalytic converter is very expensive, before you replace it, check issues with other components like the oxygen sensor, the muffler, the exhaust pipe, the manifold, or the oxygen wiring. 

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The catalytic converter is responsible for preventing any harmful gases from reaching the environment by converting them into less harmful gases. Any problems with the catalytic converter might affect your vehicle's drivability and results in failing the emission test.

One of the quickest ways to keep an eye on the catalytic converter performance is monitoring any error codes coming with the check engine light illuminating. One of the very common error codes indicating an issue with the catalytic converter is the P0420 code.

This article provides you with all you need to know about how to fix the P0420 code. We provide you with a list of possible causes and symptoms to keep an eye on when dealing with a P0420 code. We also list rough estimates for the expected repair costs to get rid of the P0420 code. 

What is the P0420 code definition?

The official definition of P0420 is “Catalyst System Efficiency Below Threshold.” If you don't have advanced mechanical skill sets, you can find it challenging to understand the meaning of this code unless you understand the system itself and how it works.

Therefore, look at the following section below to get a better idea of what it means when the catalyst system efficiency is below a certain threshold. 

What is the catalyst system efficiency, and how is it linked to the P0420 code? 

The catalytic converter is responsible for reducing the harmful emissions from getting released to the environment. As the name suggests, the catalytic converter converts harmful emissions into less harmful gases that are OK to be released to the environment without causing health issues. 

 This converter is a major component, and it's very important to work properly so your vehicle can at least pass the emission test and not harm other people.

The catalytic converter is connected to two different sensors responsible for monitoring oxygen in the engine. One sensor usually fluctuates, which is the one located upstream. On the other hand, the downstream sensor has readings that are usually steady.

These two sensors should not operate with the same reading. If they have the same reading, it indicates that the catalytic converter is not working with the required efficiency, and that's when it throws the P0420 error. You will see it displayed as a check engine light emanating on the dashboard. 

What are the primary causes for the P0420 code?

When the PO420 code is triggered, the problem might be linked to one of the following causes:

In some vehicles, the car might call for unleaded fuel, and when using the lead fuel, your engine will trigger a P0420 code. Keep in mind that this condition is unlikely to happen.

  • Problems with the oxygen sensor that might be failing or damaged 
  • Some issues with the connectivity of the downstream sensors 
  • Problems with the engine coolant temperature sensor 
  • Problems with one of the exhaust system components, including the exhaust pipe, muffler, exhaust manifold, and catalytic converter 
  • Malfunction in this park timing 
  • Problems with the oxygen sensors reporting showing that they record the same number 
  • Very high fuel pressure or leakage in the fuel injector 
  • Problems with soil contamination 
  • Issues related to engine misfiring 

How do I know if it’s the P0420 code? Symptoms of the P0420 code

Since the P0420 code is linked to sensitive component, it is crucial for you as a driver to keep an eye for any symptoms indicating that the catalyst system efficiency is not at the proper level by monitoring symptoms of a PO420 code. 

Luckily and as experts in the automobile field, we collected some of the common symptoms that you might look for to confirm that your vehicle suffers from a P0420 code, including: 

  • Whenever you warm the vehicle up, you'll still deal with power loss
  • When you try to accelerate over 30 to 40 mph, it won't happen because your vehicle might experience some slow acceleration
  • You might smell some sulfurous smell coming from the exhaust system
  • You will notice a significant reduction in the fuel economy
  • Also, you might experience some issues with a reduction in drivability, rough idling, hard shifting, and sometimes misfiring
  • Your vehicle will not have the required air to fuel mixture ratio
  • You might see some blue smoke coming from the exhaust system as your vehicle might be burning oil 
  • Obviously, with a P0420 code, your vehicle will not be able to pass the emission test 

P0420 code diagnostics: How do I fix the P0420 code?

The P0420 is a generic error code that occurs in any vehicle despite its type or model year. However, fixing the P0420 code requires certain steps that might differ between a vehicle and another.

Usually, vehicles like VW, Subaru, GMC, Ford, Honda, Chevrolet, and Toyota might have a very similar procedure to fix the P0420 code. 

  • Confirm it's a P0420 code 

Before we move into the details on fixing the P0420 code command is very important to make sure that the problem exists in the 1st place. Therefore, the first step here is to use an OBDII tool scanner and connected it to the vehicle to check whether the P0420 code is there or not.

There are 20 available OBDII scanners that you can purchase. As a driver, you don't have to on the most advanced in most expensive OBDII scanners because those scanners are more for professional mechanics who use them daily. Using a simple OBDII scanner, you will be able to scan the codes and get the error code. On the screen, your scanner should say P0420 code, and it will display a text saying, “Catalyst System Efficiency Below Threshold.”

  • Clean or replace the catalytic converter 

Sometimes fixing the P0420 code can be as simple as cleaning the catalytic converter. There are plenty of catalytic converters cleaners available on the market that you can review and take a look at the best one to serve your needs.

Keep in mind that cleaning the catalytic converter might not be the permanent solution which means that even if it helps you temporarily, you should still track the Real culprit causing the problem.

Before you attempt to replace the catalytic converter, which is a very expensive component, it is important to ensure that the problem is not coming from another component. Sometimes installing a new catalytic converter will not solve the issue because you will still cause some damage is to the new catalytic converter when connecting it to the faulty component.

  • Fix other damages 

Some other common damages that might be causing the P0420 code are listed below:

  • Misfiring
  • Fuel injectors leak
  • Oxygen wires damages
  • Problems with the connections to the oxygen sensors
  • Faulty temperature sensor in the engine
  • Bad catalytic converter
  • Bad exhaust pipe
  • Faulty manifold
  • Malfunction muffler 

Can a bad O2 sensor cause a P0420 code? 

A bad O2 sensor can easily trigger the P0420 code here at. This sensor might wear out over time, requiring replacement. Your vehicle's only way to tell you that the O2 sensor has a problem is by triggering a check engine warning light. 

Whenever you see the check engine light, the problem is coming from issues with the oxygen sensor. Therefore, you need to use an OBDII scanner to read through any existing codes and make sure that 100% confident it's coming from the oxygen sensor. 

How much does it cost to resolve the P0420 code?

It's very challenging to provide a fixed range for how much it will cost you to get rid of the P0420 code because the problem can be linked to various causes. In general, based on our experience in the field, we've seen that fixing a P0420 code might cost you somewhere between $500 and $1000. This cost only covers parts and does not include labor, ranging from $100 to $200.

Keep in mind that if your problem is related to a failed catalytic converter, you deal with a different story that might cost you the entire vehicle. Catalytic converter cost can approach about $3000 if not more in some modern vehicles.

Can I drive my car with a P0420 code?

Although the P0420 code is not life-threatening, you should immediately take care of the problem to prevent major complications in the catalytic converter, which was one of the most expensive components in your vehicle.

The hard thing about the P0420 code is that you might not even notice the issue if you don't drive on the highway speed because the drivability might not even change. However, drivers who are usually taking the highways might see a significant impact on vehicle handling, especially when they try to accelerate.

As experts, we recommend that whenever you see any symptom of a P0420 code to take care of the problem immediately to avoid costly repairs. 

Is it worth fixing the P0420 code? 

It depends. If the problem is related to a loose connection between the catalytic converter and its oxygen sensors, fixing it and tightening any loose connections is worth fixing.

On the other hand, if the problem is related to a major issue in the catalytic converter, you need to step back and evaluate the situation.

Compare total repair costs to your vehicle's overall value. Calculate how many repairs you'll need, and it is called regular maintenance in your calculations. For example, if you know that your car is due for a tire rotation or maybe an oil change, consider adding these small costs because they will pile up at the end of the day. 

Once you have a total number, compared it to the value of your vehicle and see if fixing this problem will increase the value of the vehicle or not. Also, consider comparing with high repair costs approach about 75% or more from your vehicle's value if that happened; it is never worth spending a penny trying to fix this car because it's not worth it. Instead, you should think about selling your vehicle to Cash Cars Buyer, who guarantees to pay the top dollars for this car and retailing. 

If you would like a better idea about how Cash Cars Buyer works, we advise you to give us a call at 866-924-4608 or reach out to our home page to get additional details. 


When your vehicle's check engine light illuminates, you must not ignore the problem, and you must take immediate action to see what is causing it. Sometimes the check engine light might Luminate due to a simple issue like a loose fuel cap and can be linked to complicated problems like complete engine failure.

The P0420 code is linked to a critical component which is the catalytic converter. This catalytic converter is very expensive. Whenever you see any indication showing that the catalytic converter suffers from something, you must immediately take care of the problem to avoid very expensive repair costs.

If preparing the vehicle to clear the P0420 code will cost you a lot, we advise you to think and evaluate the situation about whether it's worth selling your car to Cash Cars Buyer or not.

Cash Cars Buyer is one of the top-rated car removal companies in the nation that guarantees to pay you the top dollars and will provide you with free towing despite your living location around the United States. 

The best thing about Cash Cars Buyer is that our process is very straightforward and doesn't take more than a couple of days to get your vehicle removed safely and for the most money. 

To learn more about our process and our team, you can reach out to us by giving us a call at 866-924-4608 or visit our home page click on the free instant online offer.

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