P0420 code — how to fix error fast it

If you have a “check engine” light glowing up on the dash and plugging the OBD2 scanner into the jack reads problem code P0420, you’ll probably take it as a cause for concern. It happens more often than you think, so our team will help you understand the P0420 code meaning and why it may show up in a car.

We’ll also tell you all about the common reasons behind it, as well as how to erase it and finally fix it, so let’s get right into it.

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What does the P0420 code mean?

The P0420 code is a diagnostic trouble code that indicates a problem with the catalytic converter. If you don’t know what it is, let’s just say that the catalytic converter is a key component for reducing gas emissions within your exhaust pipeline.

So, not only does it make your vehicle “greener”, but it also helps optimize fuel efficiency. By that, you can probably guess that if this component gets out of bounds, the engine’s combustion may get disrupted too and that could eventually lead to some engine-related issues.

We know it sounds a bit grim, but that’s exactly why we’ll cover the potential causes of the code. We’ll also discuss how you can recognize the issue, fix and remove the code, and eventually apply a few counter-measures to never get this code to show up again.

What causes the P0420 code?

Several factors can cause the P0420 code to pop up on the dash. One of the most common reasons is a faulty or damaged catalytic converter. However, there are a few other problems that could initiate the code and our team wanted to make sure you know about all of these:

Oxygen sensor issues

The P0420 code can occur if there’s a problem with the oxygen sensor. The oxygen sensor measures the oxygen levels in the exhaust system and reports back to the engine control unit. However, most modern cars have two oxygen sensors – one before the catalytic converter, and one post-cat sensor. 

The latter also helps determine the efficiency of the converter, so if it goes bad, chances are that’s what’s causing the P0420 code.

Engine misfires

Engine misfire is yet another common reason behind this code and it can occur for a number of reasons including faulty spark plugs or ignition coils. Once this happens, the engine will lose the momentum of the RPM level and it leads to unburnt fuel retracting through the exhaust pipe. 

As you can probably guess by now, that can damage the cat since it may be overwhelmed by all that fuel and it could lead to this problem code.

Fuel system issues

Issues with the fuel system can also cause the P0420 code. If there’s a problem with the fuel injectors or fuel pressure regulator, it can cause the engine to run too rich or too lean, resulting in a catalytic converter malfunction.

Which car models does the P0420 code affect the most?

While most cars will show up with the code P0420 for similar reasons, some car brands seem to be affected by the issue a bit more frequently than others. It’s mainly due to root causes of the issue like engine misfires or poor quality of the original oxygen sensors that are prone to wearing off prematurely. Here’s our list of the most commonly reported car brands that showcase this issue:

• Honda Accord: We found out from several mechanics that this model suffers from the code mostly due to low-quality oxygen sensors in cars that use a system with two sensors.  
• Ford Focus: Improper air intake and issues with the exhaust system or tubes are often the culprits behind the code in Ford Focus vehicles.
• Nissan Altima: This model has frequent reports of the code P0420 due to failing catalytic converters.

How to diagnose the P0420 code

Diagnosing the P0420 code can be tricky, so our best piece of advice is to rely on an experienced mechanic or car electrician to do the drill. However, there are a few steps you can take to narrow down the causes before taking it to a mechanic:

Use a diagnostic scanner

A diagnostic scanner can help you identify the specific problem with the catalytic converter. The scanner will read the code and provide a description of the issue. In this case, it may read the code and give you a bit more background on what’s causing the trouble.

Perform a visual inspection

Inspecting the catalytic converter for damage such as dents, cracks, or leaks can help you identify the issue. If there’s physical damage to the catalytic converter, you may need to replace it to fix the P0420 code.

Check the oxygen sensor

Checking the oxygen sensor can help you determine if it’s the cause of the code. You can use a multimeter to test the sensor’s voltage output to determine if it’s within the manufacturer’s specifications.

How to fix the P0420 code

Fixing this code will depend on the underlying issue. Because of the precious metals used within the catalyst chamber of the converter, it’s a bit expensive to replace for most car models, even ranging from $2000 to $3000.

At least you can hopefully diagnose the issue right, so replacing the cat may not be the only solution to the problem and you’ll have the following options at your disposal:

Replace the catalytic converter

If the catalytic converter is damaged, replacing it is the best option. The cost of a new catalytic converter can vary depending on the make and model of your car, and it’s essential to always use a proper replacement based on the proposed recommendations of the manufacturer.

Replace the oxygen sensor

If the oxygen sensor is faulty, replacing it could solve the issue especially if it’s that post-cat sensor we already mentioned. Once you replace it, the new sensor will once again pick up the normal function of the catalytic converter and provide this information to the ECU, and the code will likely disappear.

Repair the fuel system components

If there’s an issue with the fuel system, such as a faulty fuel injector or fuel pressure regulator, repairing or replacing these components can fix the P0420 code.

How to prevent the P0420 code

Preventing the P0420 code can help you avoid costly repairs and keep your car running smoothly. This way you can both avoid the high cost of a new converter and keep the original converter within the exhaust system for the most optimized engine performance. Let’s take a look at some of the main prevention options:

• Regular maintenance
• Driving responsibly
• Taking measures to prevent catalytic converter theft
• Using high-quality fuel

Another reason why your O2 sensor posted after the converter may pick up a faulty operation and trigger the code is if the converter is actually missing. Sadly, this is a common situation these days since there are more and more catalytic converter thefts going on.

It can’t be helped, since the cat’s catalyst chamber contains precious metals like palladium, rhodium, and traces of platinum. While it makes a precious prize for thieves, you can apply a few methods to protect it by imprinting your VIN or painting the converter so it can easily be identified. Most thieves will back off from stealing it in this case, and you can also install an anti-theft device or powerfully weld the bolts so it’s more difficult to snatch it.

Our take

Ultimately, the P0420 code is a common issue that many car owners face. At least you are now full-prepared to diagnose it and tackle the issue before it affects your engine’s performance, as well as avoid having to replace the converter as one of the worst-case scenarios. While you should be able to set up the initial diagnosis yourself with the help of the OBD2 scanner tool, it’s always best to rely on a trusted mechanic in case the problem persists.