What To Do With A Code P0420

Codes are numerous on our diagnostics tools and, when we don’t have them all memorized, finding the root of our engine issues becomes all the more difficult. That’s okay. We put together a series of code explanations and you just happened up code P0420, more than likely because this is the code you found on your screen. 

So let’s take a look at code P0420, what it means, and how to take care of it. 

Code P0420

What does a code P0420 mean for your engine? In short, it means that bank 1 of your catalytic converter isn’t performing efficiently. 

Even if you are new to the world of engines the odds that you have heard the term catalytic converter are pretty high. This is because they seem to be in high demand with thieves, which is why they have become a household term. 

What Is A Catalytic Converter?

Catalytic Converter under car.

When you think about what this part does it’s not a surprise that a catalytic converter is so important. It converts the harmful gasses your engine would likely emit into less harmful fumes so that the world doesn’t get that polluted. 

Gasses like carbon monoxide, hydrocarbons, and nitrogen oxide are what your engine would be released into the air if it weren’t’ for the way this functions. While carbon monoxide is deadly to all air-breathing animals, the other two cause smog, and nitrogen oxide produces acid rain on top of that. 

It does this by using two natural catalysts; platinum and palladium. But introducing these into the element carbon monoxide into carbon dioxide, hydrocarbons into carbon dioxide and water, and nitrogen oxides revert back into nitrogen and oxygen. 

Other ways your engine might be giving you hints that the catalytic converter isn’t working well are as follows. 

Check engine light. Car dashboard in closeup
  • Check engine light comes on
  • Engine lacks power
  • Fuel economy has gone down
  • A smell of sulfur or is emitted from our exhaust

Is It Important?

Yes. Not only will leaving a poorly functioning catalytic converter omit harmful toxins in the air, which is not a great way to help your community, you will also be doing much more damage to your engine, which would make repair costs increase. 

No one will be in danger if they drive the vehicle with the P0420 code but, again, it can cause further damage to your engine. 

Getting To The Root Of It

Code P0420 doesn’t come with a quick fix because there are so many reasons why you might be getting that. Here they are. 

Mechanic repairman at automobile car engine maintenance repair work
  • Using leaded gasoline
  • Misfire (one or more cylinders is not working correctly)
  • Air-to-fuel ratio is rich or lean
  • Exhaust system leaks
  • Damaged air-fuel sensor
  • Damaged oxygen sensor
  • Faulty or old catalytic converter

One quick thing, if you are using leaded gasoline you need to use unleaded otherwise you can do damage to your catalytic converter. Also, if your engine is misfiring then replacing the converter won’t work for long because the misfirings will wear it down. 

How To Find The Problem

Step One

Man inspection holding Battery Capacity Tester Voltmeter.for service maintenance of industrial to engine repair.In Factory transport automobile automotive image

Double-check that you have only a P0420 code. Not only is it in your best interest to check again before you start sifting through your engine parts other codes might come up and those need to be remedied first before you begin on the P0420. 

Step Two

First, check your exhaust system completely. Make sure you don’t see any indications of a leak or leaks. This could be the problem. You want to be especially attentive to the pre-catalytic converter exhaust pipes, the exhaust manifold, and the gaskets. 

Step Three

If you find any leaks, repair them. When you are done clear the code on our tool and do a few test cycles. Move on to the next step if the P0420 appears because that means the problem goes deeper. 

Step Four 

Time to get the digital multimeter to check the voltage reading of the downstream O2 sensor, which tracks the gasses that come out of your catalytic converter.  If the digital multimeter bounces from 0.1V to 0.9V then you have a bad converter and it needs to be completely replaced. 

Things To Look Out For

Catalytic Converter
Image Source : wikipedia.org

Before you run out and buy a new catalytic converter it’s important to make sure there aren’t any other issues that could be causing it to fail. For example, a code P0300, P0301, P0302, P0303, P0305, P0306, P0307, or P0308 all indicate misfiring cylinders, which we discussed earlier. 

Or, you might find codes P0174, P0171, P0172, or P0175, which indicate that your engine is running rich or lean, and this can do a great deal of harm to a new catalytic converter. This is why we always recommend clearing up other issues first because a new converter won’t last long with these issues. 

How To Fix It

Besides the advice we gave in the How To Find The Problem section we are out of repair instructions to give because, anything else, will require much more work than we can explain here. 

If you feel up to the challenge, then wonderful. We have a list of explanations and fixes that can help you get started with those P0300s. Otherwise, find a good mechanic for these issues. 

The Cost Of Repair

Mechanic and car repairing in garage

Like any other issue that could be caused by one problem or ten, it is nearly impossible to give you an answer on what it costs to make the repairs needed for this code. Luckily, you are interested enough to have come this far so you could save some money by fixing what you can yourself. 


While driving on a P0420 code might not put you or your passengers under any possible threats other than breathing in noxious fumes leaving any needed repairs unattended will be worse the longer it goes unfixed. Which is why you are going to take care of that issue right away and prolong the life of your engine.

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