What Does Code P0420 Mean and How Do You Fix It?

It can be tough sometimes to know what’s going on when you just have an obscure code to work from. In this article we are going to look at the P0420 code to explain what it means, what can cause it and how you can fix the issue.

What Does the Error Code P0420 Mean?

The exact definition of the P0420 code reads "Catalyst System Efficiency Below Threshold (Bank 1)" but what does that mean? Basically this means that the vehicle's Engine Control Module (ECM) has detected an error in the catalytic converter's efficiency on Bank 1.

This ECM uses two O2 sensors one at the front and one at the back of the catalytic converter. These sensors measure the efficiency of the part and if they deem it to be low an error warning will be logged. Essentially this indicates that there may be an issue with the catalytic converter or another related part.

What Is a Catalytic Converter?

If you grew up during the 1970s and '80s you may recall occasionally driving around in cars with the windows down and smelling a sulfur rotten egg smell from time to time. After exclaiming “What is that smell?” someone in the car likely enlightened you to it being a catalytic converter.

This simple answer doesn’t mean much so let's explore just what a catalytic converter actually is. Basically catalytic converters are devices that capture the emissions from the burning of petroleum. Once captured these fumes are stripped of carbon monoxide, nitrogen oxides and hydrocarbons.

The remaining emissions are then released from the catalytic converter in the form of carbon dioxide (CO2) and Water (H2O). These emissions of course are far less harmful to the environment meaning that the fuel burning process is cleaner.

How Do Catalytic Converters Work?

There are many different types of catalytic converters but they all work along the same principals. Essentially inside these devices there are chemical elements that are used as catalysts. There are reduction catalysts and oxidation catalysts.

These catalysts are metals such as platinum, rhodium or palladium which by the way are not inexpensive. This often means that replacing the catalytic converter is not cheap. The metals are often coating ceramic structures and will trap and react with the carbon monoxide, nitrogen oxides and hydrocarbons as they pass through the device.

Firstly reduction catalysts such as platinum or rhodium act upon the nitrogen oxides ripping away the nitrogen atoms from the compound. As an example when nitrogen dioxide (N02) passes over these catalysts the nitrogen (N) is ripped away leaving only the two O atoms which for those who might not know is simple oxygen.

The next phase is the oxidation catalysts which might be platinum or palladium. These catalysts with the help of the extra oxygen from the reduction phase take care of carbon monoxide CO and hydrocarbons. Instead of removing atoms they actually force a bond between the O2 and CO molecules turning oxygen and carbon monoxide into carbon dioxide (CO2).

Although excess CO2 is still not great for the environment it is much more preferable to carbon monoxide which can be lethal. Poorly maintained gas burning heating systems for example might produce excess carbon monoxide in your home. The accumulation of this is poisonous and can kill.

What Can Cause a P0420 Error Code?

The obvious and of course most common cause of the error code P0420 would be related to the catalytic converter which may have developed an issue. Other parts also may be at fault as well and as they will likely be cheaper to repair than the catalytic converter you should check on these first.

Faulty 02 Sensor

As mentioned two O2 sensors are involved in monitoring the efficiency of the catalytic converter. If one or both of these sensors is not working correctly then you will not be getting an accurate reading on your emissions.

Exhaust System Leak

The catalytic converter is essentially part of the exhaust system of your vehicle so if a leak develops in this area it can be allowing exhaust fumes to escape. As a result the system is not working correctly and the O2 sensors will be getting flawed readings.

Oil in the Catalytic Converter

Oil is everywhere in the engine but sometimes it finds its way into places it should not. When it does this can lead to poor engine performance and potentially burning oil. Fuel is supposed to burn but not the oil. Burning oil causes more noxious fumes and also blue smoke from the exhaust.

Wrong Type of Fuel

This can happen more often than you might think but using the wrong kind of fuel in your engine can be the cause of a P0420 error code. Gasoline and diesel burn differently so the emissions from them are also very different.

If you fill up with the wrong fuel you can get the P0420 code but you will also get a lot of others as well as this can do major engine damage if not dealt with quickly.

Engine Misfires

The O2 sensors either side of the catalytic converter work off the idea that the engine is working to full efficiency. However, if you have issues that are causing a misfire then you might get the P0420 error code.

Misfires occur when one or more of the engine's cylinders are not firing as they should. Each one requires the right mixture of fuel and oxygen to which a spark is added to create a small explosion. A lean air/fuel mix can cause the cylinder not to fire as can a faulty spark plug.

The result of a misfire is that the engine does not run smoothly and may create more emissions as a result. This can cause the 02 sensors to record a P0420 error code.

Aftermarket or Improperly Fitted Catalytic Converter

A catalytic converter can be in perfect working order but a P0420 error may still be recorded in certain circumstances. If you need to replace the part and the replacement is not as efficient as the stock model that came with the vehicle you will get this code. Any fault in the catalytic converter's placement can also cause an issue.

Symptoms That Accompany a P0420 Error Code

You won't know that the error is related to a P0420 code until you scan your Engine Control Modules memory. Therefore in this section we will look at some possible symptoms to watch out for when it comes to this type of issue.

Check Engine Light

Any error relating to the engine and exhaust system will be recorded in the ECM. When this happens it will also likely trigger a check engine warning light. This will be your first indication that you may need to scan and find out the error code.

Misfires

Again we mention the misfires because if your engine is experiencing misfires then something is not working as it should. You may hear a noticeable change in engine noise, see smoke from the exhaust and possibly smoke from under the hood. A vibration may also be noticeable as you drive along which can be caused by the misfiring cylinder.

Drop in Fuel Economy

Issues that can trigger a P0420 error code can also result in higher fuel usage. If the engine has to compensate for an issue it might use more fuel than is normal. A marked drop in fuel economy can be a big indicator of an issue.

Fixes for a P0420 Error Code

As mentioned there are a number of possible causes for this particular code to be recorded so potentially there are a lot of fixes to the issue. As we do not know for sure what the issue is until we take a closer look it is wise to not just start replacing parts.

Below is a list of possible fixes once you find the issue:

• Clean the Catalytic Converter
• Replace the Catalytic Converter
• Replace O2 Sensors
• Replace Damaged Wiring
• Replace Engine Sensors
• Fix Fuel Mixture Issues
• Replace or Clean Spark Plugs
• Replace Engine Control Module

There are more fixes than this but without knowing fully what has gone wrong it is hard to assess what you should do.

It’s All About the Other Codes

The P0420 code alone points in the direction of the issue but we still need more details. It is likely that another code will be reported around the same time that can help you diagnose the issue. As an example codes P0300 – P0308 refer to misfires in the engine. They may relate to a specific cylinder which may need repairing.

Issues with fuel/air mixtures may produce codes such as P0171 – P0175. These can indicate issues like faulty engine sensors, fuel injectors or fuel pressure issues. Essentially look for other codes which can help you pinpoint where your issue may actually be before you replace or attempt to fix anything.

What Can Damage a Catalytic Converter?

As we explained earlier in the article the catalytic converter turns harmful engine emissions into less harmful gases. The corrosive nature of those harmful gases does take its toll and if the engine is not working properly this can be worse.

In the list below we will mention some of the things that can cause damage to the expensive catalytic converter:

• Engine Misfires
• Burning of Engine Oil
• Exhaust Leaks
• Intake Leaks
• Rich Fuel Mixture (too much fuel)
• Lean Fuel Mixture (too much oxygen)
• Bad Engine Control Module

Conclusion

The P0420 error code indicates an issue with the catalytic converter's efficiency which may mean you are producing bad emissions from your exhaust. This can be harmful to the car over time and it’s not good for the environment either.

This code can point you in the direction of a problem but it will also have associated codes that can help you discover the issue more precisely.

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