P0420 and P0430 Codes - Diagnose & Fix Engine Codes - 1A Auto (2024)

Learn to Diagnose and Fix Catalytic Converter Issues

So, your check engine light is on, andyou’vegot either a P0420 or P0430 code, on your OBD-II scanner.This “Catalyst system efficiency below threshold”code is triggered when your car’s computer (thepowertrainor engine control module [PCM or ECM]) detects acatalytic converterissue.

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

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

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 codeindicateswhere the problemtriggering themislocated:

• P0420, “Catalyst System Efficiency Below Threshold (Bank1)”means the problem causing itis 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 pollutantsresulting from thecombustion processso they’re less toxicand less releaseby the time they emit from the exhaust.

A code P0420 or P0430 meansa 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—onethat’sclogged, andonethatisn’t.She explains that thecloggedcatalyticconverter’sfilter, often referred to as“honeycomb,”preventsthe correct amount ofair 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 and the damage has been 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-runningengineor 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 requiresreplacing the entirecatalytic converter.

Other causes can include:

• A faulty or failingo2 sensor
• Catalytic converter leaks
• Exhaust,intake,ormass airflow(MAF)sensorleaks or other issues
• Faulty or failingspark plugs,ignition coils, or a resulting misfire
• Fuel injectorleaking toomuch fuel
• Faulty mechanicalorinternalengine components
• Lack of regular maintenance, such as not changing air filters, or oil or coolant getting into the exhaust because theyweren’t changed

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 EfficiencyOrBad 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 fuelfrom a leakyfuelinjectorcould be what caused the O2 sensor to go bad in the first place. He said they can also sometimesget stuck. Even if it’s clear that your catalytic converter needs to be replaced due to a clogged filter, Andy says it may be best to replace your O2 sensors at the sametime, rather than take a chance.

Ifthedownstream O2sensor detects a limited amount of carbon dioxide, which means thecatalyticconverterisn’t able toeffectivelybreak down pollutants, that’s when a P0420 code is triggered.

To learn more about the difference between downstream and upstreamO2 sensors from one of our mechanics, watch this video:

O2 Sensors: Is It Upstream or Downstream?

How Do I Diagnose and Fix Codes P0420 and P0430?

Our mechanic Andy says the reason the2009 Chevy Malibuhe works on in the video below has a P0420 code isthatthe two o2 sensors’readings areoff,anddetect an issuein 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

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 catalyticconverter’sfilter, or screen,forclogging

Andy’s catalytic converter filter is plugged up, and he compares it to the one in anew 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’llneed toreplace your catalytic converter, he recommends checking out a few other items before replacing it:

• Make sure your engine is running properly.
• Check for anyexhaustorintakeleaks, including any that arepast theMAFsensor, or for any other intake orMAFsensor problems.
• Make sure yourspark plugsare in good shape and working rightin order to avoid amisfire, 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 andfilterchanges,your carismore likely to experienceissues that triggertrouble 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 clogged catalytic converter.
• If you have any otherOBD-II codes, check those before replacing your catalytic converter!Catalytic converters are not cheap parts, so you shoulddefinitely checkout any other codes your car has before buyingone in case there’sactually somethingelse causing the problem.

How Much Does It Cost to Fix Codes P0420orP0430?

Depending on your vehicle, catalytic converterscan range from a low of about $110toupwards of $1,000. And that doesn’t include labor costs if you’re planning tohave 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 canfind out how much it will cost for your year, make, and modelon 1aauto.com.

Shop parts featured in this article:

• Catalytic Converter
• Oxygen Sensors
• Exhaust
• Mass Air Flow Sensor
• Spark Plugs
• Ignition Coils
• Fuel Injector
• Air Filters
• Scan Tools

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