How to Fix Code P0420: The Quick Rescue Guide

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P0420 showing on OBD scanner

You don't want to see the P0420 code when you scan your vehicle with a diagnostic tool. This code means there is an issue with the catalytic converter. If this happens, don't hesitate to address the problem as soon as possible! We'll go over what it means by looking at some of the symptoms, causes, and fixes for this problem.

What is the P0420 Code?

The P0420 code is a generic OBD code stored in your car engine control module and triggered by a faulty catalytic converter. This can cause problems with your car's emissions and impact its performance. 

The catalytic converter's job is to break down and transform toxic pollutants produced by your car during the combustion cycle into less damaging gases, so if it's not working properly, it can cause all sorts of issues.

Causes of P0420 Code

There are lots of potential causes for this issue. Some of the most common and uncommon problems are:

  • The catalytic converter has been damaged
  • Faulty oxygen sensor
  • Damaged wiring and lost connections
  • Exhaust leak

1. Faulty Catalytic Converter

A P0420 code will most likely appear if your catalytic converter is malfunctioning. This is the case because the converter is in charge of converting toxic emissions into less harmful gases. As a result, if it's damaged, it might create a slew of problems with your vehicle. These circumstances may cause the catalytic converter to fail:

  • Misfire
  • Inadequate oil/ fuel ratios
  • Contaminants like oil and cooling

2. Problem with the Downstream Oxygen Sensor

Another common cause of a P0420 code is a problem with the downstream oxygen sensor. This sensor helps monitor the efficiency of your catalytic converter, so if it's not working correctly, you'll likely see this code pop up.

3. Damaged Wiring and Lose Connections

The P0420 code might also be caused by faulty wiring or loose connections in your exhaust system. Check all of your wiring and connections for any signs of damage or corrosion to avoid spending money on expensive repairs.

4. Exhaust Leak

An exhaust leak can also cause the P0420 code. So, be sure to check your entire exhaust system for any leaks.

How to Fix the Code P0420?

The catalytic converters fill up with carbon over time and won't be as efficient at filtering out your exhaust gas. The best solution is to take it off - but if you're not comfortable doing that type of work yourself or know someone who can do it quickly in an emergency, then go ahead and visit a mechanic! 

Meanwhile, you can also try to repair or clean a catalytic converter on your own by following these steps:

Clean the Catalytic Converter Using Thinner

Having a problem with your catalytic converter will be a pain in the ass as you will need to buy one for replacement. However, there are many ways to prevent spending too much by cleaning the catalytic converter on your own. 

This will not guarantee that your car will pass the emissions test, but it does start. One way to clean the converter is by using thinner. All you need is a container, safety gear, and of course, the catalytic converter. You will also need an OBD tool, so you can check if the code has been cleared after doing this remedy. 

  • Pour a gallon of thinner into the gas tank.
  • After that, run the engine at about 2500 RPMs for 30 minutes to run the cleaner through.
  • Doing this can burn impurities out of the catalytic converter and make it work again.

Remove and Soak

If cleaning the converter does not work, don't worry, as there is another way you can try: removing the catalytic converter and soaking it all night. Although this is an extra challenge, there could be a higher chance that your catalytic converter will work than the cleaning method. And to get started:

  • Remove the catalytic converter. This is located under your car, so it will be a bit difficult.
  • Once removed, get a bucket and put some detergent in it.
  • Fill the bucket with water until the detergent is dissolved.
  • Get the catalytic converter and put it in the bucket with soap.
  • Leave it for the whole night to remove all the crud that's inside it.
  • The catalytic has a little honeycomb inside that clogs quickly, so cleaning it will get it working again.

Note: If you replaced the O2 sensors with a catalytic converter, check for other possible reasons. Due to wiring or software issues, many vehicle owners inadvertently replace their fully functioning catalytic converters.

Symptoms of Code P0420

There aren't any definitive symptoms that will always occur when this code is triggered. However, you should look out for things if you think your catalytic converter may not function correctly. If your car experiences some of the following symptoms, you may have a bad catalytic converter that needs to be replaced:

  • Your engine will run poorly and cause more strain on other emission control components.
  • Exhaust fumes are visible in front or behind your vehicle while running.
  • An unusual smell is coming from under the hood.
  • The check engine light is on, and you receive a P0420 code after scanning your car with an OBDII reader.

This list isn't exhaustive, but it's important to note that if any of these things are happening while driving, your catalytic converter could be the cause. Please don't ignore them!

Is the P0420 Serious?

It is crucial to diagnose engine trouble codes as soon as possible. If we neglect this, our vehicle can suffer serious consequences, and catalytic converter repair costs may become overwhelming no matter what type of car you drive!

If this problem is left unchecked long enough, cracks might eventually develop, causing significant damage, including potential fires within the engine compartment.

Common Mistakes When Diagnosing Code P0420

  • The most common mistake when diagnosing the P0420 code is replacing the catalytic converter without first confirming that it's the cause of the problem. If there is a build-up of carbon on the sensors, causing the code to appear, and if there is a problem with how clean it is, this could cause problems.
  • Many people are unaware that a simple catalytic converter cleaning will solve the issue. However, this is not reliable, but there is always hope in trying. So, before jumping to conclusions and spending more on expensive repairs, it will be worth trying all the resorts you have.
  • Another mistake people make when diagnosing their cars is not checking for damaged wires before bringing the vehicle to a repair shop. This results in spending more money and wasting time. 

Code P0420 FAQ

1. Is P0420 in the upstream or downstream direction?

A P0420 code indicates that the catalytic converter is malfunctioning. There are two oxygen sensors in the catalytic converter. One oxygen sensor is placed in front (upstream) of the catalytic converter, while the other is placed at the back (rear) (downstream).

2. Is it okay if I ignore P0420?

You can ignore them as long as you don't have to pass any emission tests. It would be difficult to deceive the computer.

3. Is it true that catalytic converter cleaners work?

No, it is a quick response. Because they contain additives that remove carbon deposits from your engine's exhaust and fuel systems, using one of these products will likely improve symptoms. They won't get rid of them totally, nor will they suddenly restore your broken catalytic converter.

4. Is it possible to drive a car if I have P0420?

While P0420 does not endanger the driver, it might cause issues with driveability, such as a lack of power or slow acceleration, if left unattended. If not addressed promptly, it can also cause severe harm to other components of your car.

5. What is the location of the catalytic converter?

Your catalytic converter is placed between the exhaust manifold and the muffler in the exhaust system. The portion is either beaded or honeycomb-shaped, and it's coated in a metal catalyst, which is commonly a mix of platinum, rhodium, and palladium.


Many people find themselves frustrated when they see the code P0420 on their vehicle. This can be a real problem, but luckily, there are ways to fix it!

A malfunctioning catalytic converter causes code P0420. If you want to fix the cause of your car's lack of power and acceleration issues and prevent further damage to other components in your vehicle, it's recommended that you get this issue checked out right away.