EOBD P0420: What Does It Mean & What to Do When It Appears?

Malfunctions of a vehicle happen all the time. If someday you’ll be faced with it, the important thing is to remain calm and get to the bottom of what seems to be the issue. How exactly? Well, if you have some experience with vehicles, you might be able to figure it out by yourself.

However, if you don’t, there’s a very simple solution that will come in handy no matter if you’re a professional or just someone who wants to know what is wrong with their vehicle whenever it starts to show symptoms of malfunction.

That solution is an OBD II scanner – it’s a small device that you can keep in the glove compartment of your car so that it is always near if you suddenly need it.

One of the fault codes that might appear on the screen of the scanner is the P0420 trouble code. What does it mean? What are some of the symptoms of this engine code and how can you fix it? You will find the answers to these and more questions below.

What Does the P0420 Code Mean?

The Engine Control Unit (ECU) in your car is responsible for monitoring the emission system sensors. It is designed to optimize performance and keep the exhaust system emissions within acceptable limits. The O2 sensors in the emission system measure the catalytic converter efficiency. If the efficiency is lower than it should be, the ECU will store OBD-II code P0420.

The definition of the OBD-II P0420 code is “Catalyst System Efficiency Below Threshold (Bank 1)”.

When the ECU stores this code, it means that the catalytic converter is not operating at maximum efficiency and may need replacement.

error code p0420
Car fault code: P0420
Credit: Obdstation.com

The catalytic converter is part of your vehicle’s exhaust system – it takes the leftover fuel and carbon monoxide that haven’t been burned during the combustion process and turns them into carbon dioxide and water. Aside from this, it also reduces nitrogen oxides into nitrogen. Catalytic converters break down harmful pollutants created during the combustion cycle, reducing the amount of emissions expelled via the exhaust pipe, so you can see how important having a properly functioning one can be.

Taking a closer look at the catalytic converter, it looks a lot like the muffler in shape and size. It is made of platinum, palladium and possibly rhodium which are all rare metals. This is why the catalytic converter is so expensive to replace.

The rare metals used reduce the toxicity of the exhaust gasses. The rare metals make it efficient but also susceptible to damage if the engine or other issues are not addressed. While operating, the converter utilizes two O2 sensors – there is the upstream oxygen sensor, which is located in front of the catalytic converter, and downstream oxygen sensor, which is located after it.

If the two oxygen sensors display the same readings, it means that you have a faulty catalytic converter, which will cause higher than normal oxygen levels. This, in turn, will result in the Powertrain Control Module (PCM) triggering the P0420 code, making it appear on the OBD II scanner and the “Check Engine” light blinking (this light is also known as malfunction indicator lamp).

While chances are that you won’t notice that there is a problem with the catalytic converter, your vehicle will definitely feel the impact of the malfunction.

What Are the Causes and Symptoms of OBD-II Code P0420?

The catalytic converter’s job depends on the oxygen sensors, which is why they need to operate efficiently. For that reason, it is important to perform an oxygen sensor test before replacing or cleaning the catalytic converter. To perform the oxygen sensor test, refer to our article “How to test a bad o2 sensor” for instructions.

P0420 Causes

There isn’t one specific cause for why the P0420 diagnostic trouble code appears – the issue could be with a few different parts of your vehicle, which is why below, you will find a comprehensive list of other issues that could cause the ECU to store a P0420 code. The possible causes are as follows:

  • Muffler damage or leaks
  • Exhaust leaks at the manifold, exhaust pipe or catalytic converters
  • Other exhaust manifold damage
  • An engine misfire or skip
  • Oil leaking into the catalytic converter causing contamination
  • Defective engine coolant temperature sensor
  • Defective front or rear oxygen sensor
  • Damaged wiring or connections of the two oxygen sensors
  • Fuel injector leak
  • Higher than normal fuel pressure

As you can see there are quite a few issues that could cause the P0420 code to be stored by the ECU. It is worth taking the time to check them out thoroughly before you replace the catalytic converter to make sure that you’re repairing the right thing.

P0420 Error Symptoms

This P0420 OBD2 code indicates that there is a problem with the catalytic converter, oxygen sensors or both. When this code is stored by the ECU, the malfunction indicator lamp will come on. This is your first sign that there is a problem somewhere in the emission system.

Other signs to watch for are lack of engine power, decreased fuel economy or a rotten egg (sulphur) smell. If any of these other signs appear, then you need to get the ECU scanned. This will tell you if there is one or multiple codes stored that are contributing to the problem.

The symptoms of a bad catalytic converter which may lead to P0420 fault code includes one or more of the

  •  Check engine light comes on
  • Lack of engine power when accelerating
  • Low catalyst system efficiency
  • A decrease in fuel economy
  • Rotten egg (sulphur) smell
  • Engine misfires or skips
  • The car is hard to start

Now, let’s look a little deeper into how to diagnose and fix code P0420.

Code P0420 Test/Diagnostic

As previously stated, there are quite a few issues that could cause the ECU to store P0420 code. So where do you start to try and figure out what the problem is?

The best way to start is to refer to a list of common issues and eliminate them one by one. By doing this you will eventually uncover the issue.

Let’s take a look at some of the things that you can do to correctly diagnose the problem:

  1. Performing a visual check of the muffler, catalytic converter and exhaust pipes for any damage is the easiest place to start. If the catalytic converter has any dents, holes, discoloration or other damage, it probably needs to be replaced.
  2. Visually inspect the upstream and downstream O2 sensors for any loose connections or disconnected wires.
  3. Check to see if there are other codes paired along with the P0420. If so, check out those codes first as they can generate false readings and trigger the P0420 code.
  4. Determine if the engine is running rough or if there is a noticeable skip.
  5. Check to see if the temperature sensor indicates a very high or very low engine temperature (bad sensor).
  6. Look at the fuel injectors to see if there is excess fuel leaking around one or more of them.
  7. Run the car until it reaches normal operating temperature (may take up to 20 minutes). Take the digital multimeter and check the voltage of the rear oxygen sensor. It will produce a relatively stable .045V if the catalytic converter is working properly. If the voltage is wavering between 0.1V and 0.9V, then there is a problem with the catalytic converter, and it should be replaced.
  8. Another more sophisticated diagnostic involves reviewing the Mode 6 test results. Mode 6 tests are the actual self-tests that the ECU runs through to detect faults and monitore the readiness of the ECU. It is important that the front and rear o2 sensors pass the Mode 6 tests. If they fail, false readings could potentially confuse the OBD-II software and trigger a false P04020 code.

How to Fix Fault P0420 Code

In most cases, the failing catalytic converter is not dangerous and poses no risk to the occupants of the car. The real risk is that serious damage to the catalytic converter may occur unless the P04020 code is resolved quickly.

Since the catalytic converter cost money to repair, it’s worth taking the time to fix the P04020 code as quickly as possible.

Recommended Repairs to Fix the P0420 Trouble Code

Based on the results of the diagnostics, here are some of the common repairs that are used to fix code P04020:

  • Address and fix any other codes paired along with the P0420 first as they may be generating false readings.
  • Check and fix the muffler and exhaust pipes.
  • Repair/change the exhaust manifold.
  • Check and fix the engine coolant temperature sensor.
  • Test and replace one of the O2 sensor (or both).
  • Test the wiring or connections to the o2 sensor(s).
  • Identify and fix the problem causing an engine misfire.
  • Check and fix any leaking fuel injector.
  • Clean the catalytic converter using a cleaner that will remove any deposits or gunk in the system.

Once you’re done checking everything and are more or less sure that the problem might be fixed, take your vehicle for a test drive – only then will you know if the issue is actually solved or if you need to look at other solutions.


It is important to note that the ignition system, fuel system and air intake system can contribute to damaging the catalytic converter. As a matter of fact, these systems are the most frequent causes of the P0420 error code. For this reason, we recommend that these systems are thoroughly checked out before replacing the catalytic converter.

If after performing all the diagnostics, analysis and repairs, the P0420 code still stores in the ECU, it means that unfortunately, you will have to consider replacing the catalytic converter.

The good news is that you did everything possible to eliminate any doubt that the catalytic converter is bad. The bad news is that you have to choose between going to a professional or attempting to do it yourself. However, if we can recommend something, it’s that usually it is better to go to a mechanic, even if you have some kind of experience with vehicles. If you attempt to do it yourself, and something goes wrong, you might find yourself spending hundreds or more dollars on repairing something that could’ve been avoided.

No matter what you decide, you should remember to replace the o2 sensors (recommended) and catalytic converter with high-quality OEM replacement parts.


With that being said, we have reached the end of our short guide on the P0420 code. This is a code that appears when there is an issue with your vehicle’s catalytic converter. It’s important that this specific component is working properly since it takes care of the harmful pollutants that would otherwise exit your vehicle through the exhaust pipe.

Catalytic converters’ issues can be recognized due to the symptoms they show, including the “Check Engine” light, lower efficiency of the fuel system, excess sulfur smell, or the vehicle being hard to start. When it comes to what causes, as you could see on the list above, there isn’t one specific one – the problem can be with several different parts of your vehicle, such as damaged wiring of the upstream or downstream oxygen sensor (or a faulty sensor altogether), fuel injector leak, oil leaking into the catalytic converter, and so on.

The same goes for repairs – there isn’t one way to repair your vehicle and fix the code, mainly because different causes require different solutions. The easiest way to make sure that everything goes back to the way it should is by going to a mechanic, or someone who has a lot of experience with dealing with similar issues – they will be able to properly diagnose the cause, and, in worst case scenario, they will be able to replace the catalytic converter if the need arises.

P0420 Code – Frequently Asked Questions

Is the P0420 code dangerous?

As we already mentioned, the appearance of the P0420 code in itself is not a dangerous situation. However, just like with most vehicle issues, if it’s left unrepaired for too long, it might cause irreverseable damage to other components of your vehicle, which is why you should get it fixed as soon as you can.

How much does a catalytic converter cost?

Because of the materials they are made of, catalytic converters can be quite pricey. While the final price depends on the specific model you purchase, a new one can cost, on average, between $500 and $2,000, that is not including labor costs. Of course, you can decide to purchase a used one, which will turn out to be cheaper, but then you risk purchasing a faulty one, so just keep in mind to carefully inspect it.

How much does fixing the P0420 code cost?

It depends on what caused it to appear in the first place. The repair can cost as little as less than a hundred dollars, but it can also cost over two thousand dollars.

Can the P0420 code be caused by a faulty oxygen sensor?

Yes, it absolutely can. If the voltage of the downstream oxygen sensor decreases or fluctuates, then the oxygen levels are probably too high. If the upstream and downstream oxygen readings don’t show any abnormalities, the issue is probably with other components of your vehicle.

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