P0420 and P0430 codes: Diagnose and fix like a mechanic [how-to guide + videos]

P0420 Code
P0430 Code
Catalyst System Efficiency Below Threshold (Bank 1 & Bank 2)

So, your Check Engine light is on, and you’ve got either a P0420 or a P0430, “Catalyst system efficiency below threshold” OBD-II code that is triggered when your car’s computer (the powertrain or engine control module (PCM or ECM)) detects a catalytic converter issue. 

In this video, our mechanic explains how a catalytic converter’s efficiency is reduced when it’s clogged, which triggers one of these two error codes. Watch now: 

Check Engine Light? Catalyst System Efficiency Below Threshold – Code P0420, P0430 

You can watch more videos like this in our video library.

What are codes P0420 and P0430, and what do they mean? 

What are these “Catalyst system efficiency below threshold” codes, and how do they differ? Each code indicates where the problem triggering them is located: 

  • P0420, “Catalyst System Efficiency Below Threshold (Bank 1)” means the problem causing it is happening in Bank 1 (where your vehicle’s cylinder number 1 is located). 
  • P0430, “Catalyst System Efficiency Below Threshold (Bank 2)” means the issue triggering it is happening in Bank 2 (where your vehicle’s cylinder number 2 is located). 

According to our mechanic, “Generally, this code means that you need a new catalytic converter.” 

When your car’s catalytic converter is working correctly, it breaks down pollutants that result during the combustion process so that they’re less toxic and less are released by the time they’re emitted as exhaust. 

A code P0420 or P0430 means a catalytic converter isn’t performing as efficiently as it should. When this happens, it’s not able to break down all those harmful pollutants, and still releases them. 

In the video above, our mechanic shows us two catalytic converters—one that’s clogged, and one that isn’t. She explains that the clogged catalytic converter’s filter, often referred to as “honeycomb,” prevents the correct amount of air from getting through, which can trigger a P0420 or P0430. 

How serious are P0420 and P0430 codes—can I drive with them?

Usually, by the time you’re aware of one of these codes, it’s too late—the damage is done to your catalytic converter. If it isn’t replaced, it can get worse, becoming more plugged, which can cause weak acceleration. 

What are common P0420 and P0430 symptoms? 

  • Your Check Engine light is on 
  • Rough-running engine or lack of power 
  • Poor fuel economy 

What causes P0420 and P0430? 

The most common P0420 and P0430 cause is a clogged catalytic converter filter, which requires replacing the entire catalytic converter. 

Other causes can include: 

P0420
P0430
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Can a bad o2 sensor cause a P0420 code?

In this video, our mechanic explains how a bad o2 sensor can trigger a P0420 code.

Check Engine Light? Catalyst System Efficiency Or Bad O2 Sensor? – Code P0420/P0430 

Our mechanic Andy says that a bad o2 sensor, or one that’s reading incorrectly, can cause a P0420 or P0430. However, too much fuel from a leaky fuel injector could be what caused the o2 sensor to go bad in the first place. He said they can also sometimes get stuck. Even if it’s clear that your catalytic converter needs to be replaced because its filter is clogged, Andy says it may be best to replace your o2 sensors at the same time, rather than take a chance. 

If the downstream o2 sensor detects a limited amount of carbon dioxide, which means the catalytic converter isn’t able to effectively break down pollutants, she says that’s when a P0420 code is triggered. 

To learn more about the difference between downstream and upstream o2 sensors from one of our mechanics, watch this video: 

O2 Sensors is it Upstream or Downstream? 

How do I diagnose and fix P0420 and P0430? 

Our mechanic Andy says the reason the 2009 Chevy Malibu he works on in the video below has a P0420 code is that the two o2 sensors’ readings are off, and detect an issue in the catalytic converter. 

Watch the full video for his expert help diagnosing and resolving a code P0420: 

P0420 Check engine light how to fix it yourself 

Video from Andy The X Tech

Check your catalytic converter for leaks 

Andy recommends inspecting the back of your catalytic converter for leaks. That could cause a faulty reading from the downstream, or back, o2 sensor. 

Inspect your catalytic converter’s filter, or screen, for clogging 

Andy’s catalytic converter filter is plugged up, and he compares it to the one in a new catalytic converter, which doesn’t have anything obstructing it. 

Other items to check out 

Even though as Andy tells us, 99.9 percent of the time, a code P0420 or P0430 means you’ll need to replace your catalytic converter, he recommends checking out a few other items before replacing it: 

  • Make sure your engine is running properly. 
  • Check for any exhaust or intake leaks, including any that are past the MAF sensor, or for any other intake or MAF sensor problems. 
  • Make sure your spark plugs are in good shape and working right in order to avoid a misfire, which will only make matters worse by allowing fuel to get into the catalytic converter. If it looks like they could be causing the problem, replace them. 
  • Keep up on regular maintenance. This may sound obvious, but if you don’t properly maintain your car with regular oil and filter changes, your car is more likely to experience issues that trigger trouble codes like P0420 or 430. 
  • Inspect your o2 sensors. As Andy says in his video on P0420, it’s probably best to replace them at the same time you replace your catalytic converter if you know it’s clogged. 
  • If you have any other OBD-II codes, check those before replacing your catalytic converter! Catalytic converters are not cheap parts, so you should definitely check out any other codes your car has before buying one in case there’s actually something else causing the problem. 

How much does it cost to fix codes P0420 or P0430? 

Depending on your vehicle, catalytic converters can range from a low of about $110 to upwards of $1,000. And that doesn’t include labor costs if you’re planning to have it replaced by a mechanic. Combined part and labor prices can range from about $400 to upwards of $2,000. 

Depending on how many oxygen sensors you replace, the cost could be minimal, but you can find out how much it will cost for your year, make, and model on 1aauto.com. 

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