Check Engine Light

What Should You Do When P0420 Code Display On OBD2 Scanner?

Mike Cross
Updated Feb 12, 2022
Code P0420

A confusing and often stressful result that comes from an OBD reading is the dreaded p0420 code.

For those of you in Colorado, Washington, or a handful of other states, the number ‘420’ typically brings something more pleasant to mind, but this one has something to do with the catalyst system in your car. Here, we’ll take a look at what the p0420 code is and what you, as the driver, can do about it.

Code P0420
Code P0420

What is engine code p0420?

In short, engine code p0420 means Catalyst System Efficiency Below Threshold. If your catalyst system isn’t functioning in a proper manner, the ‘Check Engine’ light may be triggered. What you’ll want to do when this happens is get to a mechanic immediately. In most cases the P0420 code turns on the Check Engine Light because the engine is affected first and foremost.

In most cases, the light is triggered because of one of the following reasons:

  • The cylinders are burning too much oil or coolant
  • The fuel is not mixed right for your engine
  • The oxygen sensor has been triggered

One of the most interesting things about this code is that it only applies to certain kinds of cars. If you own a Chevy, Nissan, Ford, Honda, Toyota, GMC, Subaru, VW, etc. It is considered generic because it applies to all makes and models of vehicles (1996-newer), although specific repair steps may be slightly different depending on the model.

One of the biggest things you can do to prevent the p0420 code (when you are checking with obd2 scanner) from happening to you is to keep up with regularly scheduled maintenance. Depending on the climate you live in and the number of miles you drive, these maintenance checkups may need to happen more frequently than in other situations.

Largely this is because the mechanic will have the chance to check out any issue before it becomes a problem. The catalytic converter has two oxygen sensors and the crew at the shop are trained to tell if either one is not working. If the sensor isn’t fluctuating once the car has had a chance to warm up, then they can take preventative measures so that the p0420 code doesn’t happen.

How to tell if you’re going to have an engine Code P0420?

Monitor the temperature of your exhaust, particularly after a long drive.

If the temperature seems off, i.e. not 100 degrees warmer than normal, something is wrong. A P0420 code typically requires you to take more action than just swapping out the oxygen sensor (02 sensor). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.

If you suspect that anything is wrong with your catalytic converter, you can prevent the p0420 code before it happens by replacing the catalytic convertor.

How To Fix Engine Code P0420?

auto service repair maintenance

Basically, the best thing you do to fix the engine code p0420 is to do a full-on checkup of all the engine parts.

Check the catalytic convertor first.

If it looks to be all good, move on to the rest of the parts which connect to the engine. Exhaust leaks are super common, in addition to issues with the catalytic convertor.

Look at your pipes and filters, and check fluids. Because so many of these problems result from oil and fuel issues, make sure that everything is flowing smoothly. If you haven’t looked at your oxygen sensor in a while, now is a great time to do it.

In most cases, you’ll have to raise the vehicle up on a jack or in a shop in order to get at it. Keep in mind that there might be more diagnosis necessary: a p0420 code could be in addition to other issues that are going on, or resulting from those issues.

When you bring your vehicle into the shop, be prepared to have a good conversation with the person at the desk. There will likely be a bit of back and forth as you, the clerk, and potentially even the mechanic try to figure out what is going on. Trial and error might be in order, in which case the mechanic will likely give you a call later on to tell you what is going on.

If you have a decent amount of automotive knowledge, don’t be shy about sharing your thoughts with the mechanic. You know your car better than they do, and your input could prove to be highly valuable.


There isn’t much that catches you more off guard than the Check Engine light coming on as you chug down the road. Don’t worry – as long as you are proactive and get the car in for a check, you should be just fine. Many issues aren’t that big of a deal as long as you react immediately.

Hopefully this article has provided some insight to you about handling a p0420 code. If you found the helpful, please share on social media! Leave a comment below to share any crazy Check Engine light stories that you may have.

Mike Cross
Life is too short to drive with stock audio

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One comment on “What Should You Do When P0420 Code Display On OBD2 Scanner?”

  1. Great information and I'm glad there are people like Mike. I have a Code 420 that just popped up this morning. Thanks a lot.

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