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Fix P0420 yourself!

P0420 Code Explained: Catalyst System Efficiency Below Threshold

P0420 is usually related to the catalytic converter and other more serious repairs. For this reason, we recommend not driving your car until the P0420 code is resolved. Some related repairs can be done DIY but the more serious repairs (listed below) should be done by a mechanic.


What Does Code P0420 Mean?

  • P0420 definition: Catalyst system efficiency below threshold (Bank 1)
  • Issue Severity: MODERATE – Extended driving with this code can cause internal engine damage.
  • Repair Urgency: Get this code fixed as soon as possible to avoid further damage to your emission system and possible engine damage.
  • Diagnosis: The most common cause of P0420 is a bad catalytic converter, but it can be caused by anything from a faulty oxygen sensor to a rich or lean running condition, or misfires. There are many variables that could cause this code. To avoid serious engine damage, take your vehicle to a certified shop as soon as possible, or click here to save money by fixing P0420 yourself with FIXD!
  • Common Reason: Code P0420 is often related to a bad catalytic converter.

P0420 means the catalytic converter is not efficiently removing pollutants from the exhaust. Your car won’t pass an emissions test. A bad catalytic converter isn’t a hazard to the driver but it can overheat and destroy engine components.

Read more about how catalytic converter problems can hurt your engine here.

Can I Drive With P0420?

Yes, it is usually alright to continue driving in the short term with P0420. It does not pose a risk to the driver. Driving long term with this code can damage your engine and exhaust system. You don’t need to call a tow truck if you see this code, but you should try to diagnose and fix the problem as soon as you can.

You might notice a slight decrease in your car’s performance. This is because your exhaust system is not working at peak efficiency. P0420 can become more serious if there are other codes present that show there is too much fuel in your air/fuel mixture. Check to see if there are other codes like P0171, P0172, P0174, or P0175. These codes can mean that the engine is getting the wrong fuel/air ratio.

Learn more about fuel/air ratios here

How Serious Is P0420?

Code P0420 is not usually a risk for the driver. In most cases the code is not serious enough that you need to stop driving your car immediately. The longer you drive though, the more you damage your engine and exhaust systems. Diagnose and fix this code as soon as you can.

How Hard Is It To Fix P0420?

It depends on what is causing the P0420. A lot of the repairs for P0420 can be done at home. Some of the simpler repairs can be as easy as replacing an O2 sensor or some broken wiring or connectors. Other repairs are more complicated and you might need to take your car to a mechanic.

Can I Fix P0420 Myself?

Yes, you may be able to fix P0420 yourself. Depending on the cause of the code, the diagnosis and the repair might be simple or might be very difficult to do at home. You might need access to a welder if you need to patch an exhaust leak. You might also cause more damage trying to repair your problem.

You can use an OBD2 scanner like FIXD to see if you even have a real problem. You can also use an OBD2 scanner to see if there are other codes present that might be the cause of the P0420 code. The FIXD scanner also has a live data feature that lets you see real time sensor data from your car that can help you diagnose P0420 at home.

CauseRepairDIY Or MechanicResources
Misfire (p0300, p0301, p0302, etc.)This repair could be as easy as changing a spark plug or as hard as fixing a cylinder head gasketMany of the possible repairs are possible at home. See if you can rule out a harder repair before taking it to a mechanic.P0300 Engine Code Explained: Causes, Symptoms & How To Fix It
Air/Fuel mixture is lean or rich (p0171 or p0172)This repair can be as easy as cleaning the Mass Airflow Sensor or as hard as complicated exhaust repair.Many of the possible repairs are possible at home. See if you can rule out a harder repair before taking it to a mechanic.P0171 – Meaning, Causes, Symptoms, & Fixes
P0172 – Meaning, Causes, Symptoms, & Fixes
Exhaust leakPatch the damaged areas or replace partsYou can do this at home especially if you weld. There are no-weld solutions tooHow to Repair an Exhaust Leak DIY (No Welding)
Broken or damaged oxygen sensorReplace the oxygen sensorYou can try this at homehttps://www.fixdapp.com/blog/how-to-replace-an-oxygen-sensor/
Damaged wiring or connectorsReplace the damaged wiring or connectorsYou can try this at homeO2 sensor wire repair
Broken or worn catalytic converterReplace the catalytic converterThis is easier with a direct-fit bolt-on catalytic converter. It’s harder with a universal-fit catalytic converter as it needs to be welded.How to Replace Catalytic Converter in Your Car

How Do I Fix Code P0420?

You can fix P0420 at home. Before you fix P0420 though, make sure you correctly diagnose the cause of the code first. If you don’t, you could end up spending $1000s buying a new catalytic converter when that wasn’t the cause of the code.

If there are exhaust leaks or damage you will have to patch or repair them. If the damage isn’t severe, you can fix it by welding. If you don’t have a welder or don’t know how to weld, you can take your car to a repair shop for them to fix. If the damage is more severe, you might have to replace whole parts of your exhaust system.

If there is damage to one or both of the O2 sensors, the wiring, or connectors, you should replace the damaged components.

If you find that the catalytic converter is broken or the catalyst is worn, you will need to replace the catalytic converter.

Once you’ve fixed the problem make sure to clear the P0420 code and drive for about 15 minutes to see if the code returns. If it returns, you have not fixed the problem.

You can clear the P0420 code with the FIXD sensor.

What Causes P0420?

The most common reason for code P0420 is catalytic converter failure. There are other potential causes for this code though. It’s important to look for other codes that could explain why the converter failed. Misfires or an incorrect air-fuel ratio could be the causes of a failed converter.

Misfires can be the cause of a failed catalytic converter. Look for the following codes to see if a misfire happened

An incorrect air to fuel ratio could also cause a catalytic converter failure. Lean means there is not enough fuel in the ratio and rich means there is too much fuel. Look for the following codes to see if your engine is running rich or lean

The most likely cause of a P0420 code depends on your car’s model and age. Use FIXD’s tool to see what the most likely cause of this error code is for your specific car.

Other possible causes are:

  • Worn or internally failing catalytic converter (Most Common)
  • Faulty oxygen sensor
  • Faulty air-fuel sensor
  • A leak in the exhaust system
  • Misfire (the root cause of converter failure)
  • Rich or Lean air-fuel ratio (the root cause of converter failure)
  • Leaded Gasoline (the root cause of converter failure)

How To Diagnose P0420

There are four major things to check when diagnosing P0420: 

  • Make sure there are no other codes present
  • Check if the code was random by clearing the code and driving to see if it turns on again
  • Check the exhaust system for leaks
  • Check the downstream O2 sensor to see if the catalytic converter needs to be replaced

Make sure it’s the only code

If there are other codes you need to address them first because they could be the cause of the p0420 code. You can use an OBD2 scanner like FIXD to see if there are other codes present. If there are other codes present like P0300 or P0171 they could be the source of the problem. Do not replace your catalytic converter without fixing these codes first.  If you replace the catalytic converter without repairing the other code, the new catalytic converter will go bad again. 

Check to see if the code remains after clearing and driving

Sometimes the sensors in your car can detect unexpected patterns. If this happens long term, this means there is either something wrong with the car or the sensor and your check engine light will turn on. If it happens for a short period of time, your check engine light can still turn on even if the problem goes away. You should try turning your check engine light off and driving for about 15 minutes to see if it returns. If it doesn’t turn back on, this likely means there was a short term problem that you don’t need to address.

Be careful though. If the check engine light does come back on soon after turning it off with the same code, it’s likely the problem is real. You will need to diagnose and fix the problem.

You can turn your check engine light off with the FIXD sensor.

Check the exhaust system before the catalytic converter to see if there is any damage

If P0420 returns after a cycle of clearing the code and driving, you should inspect the exhaust system. Look at the exhaust system, especially upstream of the catalytic converter. Check for rust, holes, and leaks. Make sure to check the exhaust manifold, gaskets, and exhaust pipes. If you find any damage or leaks, you need to patch them.

If there are no leaks check the downstream O2 sensor

Start your car and make sure it’s at a normal operating temperature. Use the FIXD live data feature or a digital multimeter to check the voltage  of the downstream O2 sensor. When the catalytic converter is working, the downstream O2 sensor should be at about 0.45 volts. 

You Can Use FIXD's Live Data Feature To Read The Downstream Oxygen Sensor's Voltage

You need to replace the catalytic converter if the voltage is jumping between  0.1 and 0.9 volts. This means that the catalyst in the catalytic converter is worn out.

What Are The Symptoms Of P0420?

When you have a P0420 code you may notice the following: 

  • The check engine light is on
  • A lack of power from the engine
  • Decreased fuel efficiency
  • Rotten egg or sulfur smell

The reason you may smell rotten eggs is because there is a small amount of hydrogen sulfide in the fuel. Hydrogen sulfide is what gives rotten eggs their smell. A working catalytic converter should convert hydrogen sulfide into odorless sulfur dioxide. When the catalytic converter breaks, it might not do this anymore. When this happens your exhaust emits hydrogen sulfide and you smell rotten eggs.

How to Fix Code P0420

How Much Does It Cost to Have Code P0420 fixed?

Fixing P0420 can cost anywhere between $0-$2400 depending on what the cause is. Below are the cost ranges of having a mechanic fix common repairs:

  • Air fuel sensor $200-$300
  • Oxygen sensor replacement $275-$500
  • Catalytic converter $400-$2400
  • A leak in exhaust $100-$200 (if welded to repair)

It will be much cheaper if you do it yourself, but you risk causing more damage if you are not experienced.

Before trying to fix P0420 make sure you correctly diagnose the cause of the code.  If you clear the code and it doesn’t come back after driving, you don’t have to spend any money on it. If there is a small leak in the exhaust system that you can weld, you won’t have to spend much money on the repair. On the other hand, if you need to replace the catalytic converter, it is going to be an expensive repair.

We recommend using our OBD2 Code Cost Estimator to get an idea of how much it will cost to fix the issue. This tool can help you make an informed decision and avoid overpaying for repairs. 

Cost Of Catalytic Converter

Between $400-$2400 depending on the type. A direct fit catalytic converter (made for your specific model) is expensive but easy to install. A universal catalytic converter (fits a wide range of cars) is cheaper but needs welding. Some states have strict emissions laws that require more expensive catalytic converters.

A direct fit catalytic converter is one that’s made specifically to fit your exact model. If you use this type make sure to get one made for your exact car. You should be able to unbolt your old catalytic converter and bolt in the new one relatively easily. They are more expensive to buy because they only fit one unique model. You can save money by installing it yourself or get it done at a repair shop. Because it’s so easy to install, most of the cost is going to be for the parts.

A universal fit catalytic converter is made for a wide range of models. Not every universal fit catalytic converter will fit every vehicle. Make sure you get one that fits your vehicle. Since these are not made to fit a specific model you will need to install this part by cutting a section of your exhaust out and welding the new part in. While universal fit catalytic converters are cheaper, they are harder to install. If you have a mechanic install it for you, there will be more labor costs because of this. If you are more experienced this would be a good way to save money.

All cars are required to have catalytic converters that meet EPA emissions standards. Some states require cars in their state to meet the stricter CARB-compliant catalytic converters. Because of the stricter standards, these catalytic converters usually cost more. States that require CARB-compliant catalytic converters are:

  • California (all years)
  • Colorado (all years)
  • New York (1993 and newer except 1995)
  • Maine (2001 and newer)

You can find used catalytic converters for cheaper. The risk of using a used catalytic converter is it might not be in great condition. This means you might have to buy another one soon after replacing the first one.

What is a catalytic converter?

Your catalytic converter is part of the exhaust. It is between your engine and muffler.  When your engine is running, it produces harmful pollutants like carbon monoxide. Your catalytic converter breaks down these pollutants into less harmful gasses. 

Code P0420 indicates that the catalytic converter is not functioning efficiently, and is increasing the output of harmful pollutants by the vehicle.

Read our article on what a catalytic converter is and how it works

What are O2 Sensors?

 Most vehicles have two oxygen sensors (O2 sensors). One sensor is upstream of the catalytic converter and the other is downstream. These sensors help your car figure out how to balance the air to fuel ratio. It is a problem if the O2 sensors have the same reading. This is because it’s the catalytic converter’s job to reduce pollutants. If the exhaust is the same before and after the catalytic converter, it isn’t doing its job.

Your O2 sensors communicate with the Powertrain Control Module (PCM). When the O2 sensors send unexpected readings to the PCM, the PCM can trigger a P0420 code. There are many reasons the O2 sensors can send unexpected readings to the PCM. This can make diagnosis tricky.

Fixed It But The Check Engine Light Is Still On?

Check engine light sometimes need to be reset manually, check out our article:

How to Reset Your Check Engine Light | 4 Ways To Clear It (With or Without a Scanner)

FIXD Research Team

At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.

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About the Author

FIXD Research Team

FIXD Research Team

At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.

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