P0420 Code – Meaning, Causes & Solutions

In OBD2 Trouble Codes by Josh SLeave a Comment

EGR temperature sensorAre you struggling to get rid of your P0420 code? Then you have probably come to the right article to solve your problem.

The P0420 code can be a bit tricky to diagnose properly, as you think that you have fixed the fault and then 100 km’s later the light pops up again.

This code is triggered when the engine control unit is recognizing a faulty catalytic converter. It uses one O2 sensor in the front of the exhaust, and one O2 sensor in the rear of the catalytic converter. The front O2 sensor is feeling the mixture and it tells the engine control unit if the engine is running with a rich or lean mixture and adjust it. The rear O2 sensor is just a diagnostic O2 sensor for the P0420 code.

In this article, you will learn everything you have to know about the P0420 code to fix it once for all.


What does the P0420 code mean?

P0420 is triggered in the engine control module when there is a problem with the efficiency of the catalytic converter. The engine control unit uses two O2 sensors, one in the front of and one behind the catalytic converter to measure the efficiency of the catalytic converter. If one sensor is faulty and getting wrong values, or the catalytic converter is faulty, an error code will be stored.


We are getting a lot of messages from our customers about the P0420 code. It keeps coming back after several repairs and you are just wasting more and more money on the workshop repair costs. The P0420 may confuse a lot of people because there is much false information about it out there.

If the rear O2 sensor is getting faulty values, the p0420 code will be stored in the engine control unit. This could be because the catalytic converter is faulty, or a false alarm because one of the O2 sensors is damaged. There could also be a lot of other problems with your engine that are destroying the catalytic converter, and if you just install a new catalytic converter it may be damaged again.

A lot of people thinks that the P0420 code could be triggered by lean or rich mixtures. Yes, it can, but not in the way you think. Rich or lean mixtures can damage the catalytic converter and when the catalytic converter is damaged/filled with fuel, the efficiency will be low and the rear O2 sensor will see that and trigger the code Po420.


P0420 Symptoms

You will probably not have any symptoms except check engine light by just the P0420 code itself, you may have other problems with your engine that is damaging the catalytic converter that can have symptoms like rough idle, rough acceleration, misfires and hard shifting. Always fix these problems first.


P0420 Causes

The most common cause is the catalytic converter. It could be an old bad catalytic converter, but I have seen a lot of cases when the catalytic converter is new, but it’s not an original genuine part. Some cheaper catalytic converters may not be enough and in some cases, you have to buy a genuine catalytic converter from the manufacturer. I have also seen catalytic converters that have been fitted too far behind on the exhaust pipe. Because of this, the catalytic converter will not get hot enough and it will trigger a P0420 code.

  • Damaged Catalytic converter ( Most common )
  • Not-genuine Catalytic converter
  • A Faulty fitted placing of the catalytic converter
  • Damaged upstream front O2 Sensor / Faulty wirings
  • Damaged downstream rear O2 Sensor / Faulty wirings
  • Exhaust Leak
  • Intake leak
  • Oil burn (Damaging the catalytic converter)
  • Rich/Lean mixture (Damaging the catalytic converter)
  • Misfires (Damaging the catalytic converter)
  • Faulty Engine control unit (Rare)[/su_tab]


P0420 Fix / Solutions

These are just possible solutions and you should never just replace parts, it will only waste your money if you are unlucky. Check further down in the article if you want to find out how to diagnose the P0420 code properly in the same way as a skilled automotive technician is doing it. It will take some time and may require some skills, but you will not waste 1000$ on replacing a good catalytic converter.

  • Replace catalytic converter
  • Replace to a genuine original catalytic converter
  • Replace Front O2 sensor
  • Replace Rear O2 sensor
  • Repair faulty wirings
  • Fix Oil Burn
  • Fix Misfires
  • Fix Lean / Rich mixture
  • Check the data with an OBD2 scanner from Amazon
  • Replace Engine control unit (Rare)


P0420 Trouble code Table

CodeSymptomsCausesCat Damaging causes
P0420Check engine light


Rich Mixture

Lean Mixture

Motor Oil Burn/Blue smoke from the exhaust pipe

Slow acceleration
Bad Catalytic converter

Front O2 Sensor

Rear O2 sensor

Exhaust Leak

Oil Consumption

Misfires/lean mixture destroying the catalytic converter

Oil Consumption

Exhaust Leak

Intake leak

Rich Mixture

Lean Mixture



Catalytic Converter Damaged Causes

There are a few things that are known for damaging the catalytic converter, which can cause the P0420 code, here is the most common.

  • Misfires
  • Oil Consumption
  • Exhaust Leak
  • Intake leak
  • Rich Mixture
  • Lean Mixture
  • Bad ECM/PCM[/su_tab]


Important Note:

There are many things that could cause the O2 sensors or the catalytic converter to fail. You have to make sure that you fix these problems before you are replacing any parts, or they may get be damaged again. Check your DTC memory to find a combination of any other trouble codes Fix them before the P0420 Code. Make sure that your car is not burning any oil by checking the exhaust smoke, Blue Smoke = Oil, White = Water, Gray/Black = Rich Mixture.


How to diagnose the P0420 code

This is a guide to how a professional automotive technician would diagnose the P0420 trouble code. This may require some car electrical skills and you may need some tools for the task. But however, if you do not have any skills in-car electronics, you can still get some helpful information from it. You should always connect a car battery charger when you are doing any troubleshooting on your vehicle. Low battery voltage can cause other unrelated trouble codes that will confuse you and it can actually damage your control units or other parts.

1. Connect a battery charger

Connect a Car Battery charger to your battery and make sure it charges the car battery correctly. You should use a charger that gives at least 4 amperes while you are doing your troubleshooting. Lower ampere rate may cause the battery to drain out if you are troubleshooting with the ignition on.

2. Connect an OBD2 scanner

Connect an OBD2 scanner to verify the P0420 code. Some cheaper tools can only read the engine trouble code memory and may not have the capability to do any basic settings, reset adaptions and other functions that are required to fulfil the diagnose. Check the link above to see several different OBD2 scanners and learn the pros and cons of some different OBD2 scanners.

The most OBD2 scanners can only read the engine control unit. If you want to read other control units, you may get a more advanced and often more expensive OBD2 scanner.

A video on how to use an OBD2 scanner:

3. Check for other trouble codes

Check for any other trouble codes stored, that may damage your catalytic converter or any trouble codes for the O2 sensors. If there is an O2 sensor code stored, replace or diagnose the O2 sensor code first, it may fix the P0420 code also. Fix all the other codes before you are trying to fix the P0420 code.

Always note down any trouble code numbers do not erase them by accident. If you can’t get any information about the different trouble codes, you can contact us and we will give you all the information that you need.

4. Check for any blue smoke while revving

Rev up your car a bit and check carefully for any blue smoke from the exhaust pipe. Blue smoke = Oil burning and it may help to replace the catalytic converter, but it will most likely fail again really fast. Oil burning could also damage your O2 sensors and make them to not read the mixture properly.

You should always check for white, grey and black smoke. White smoke is coming from water in the exhaust pipe and grey, black smoke is coming from a rich mixture.

5. Check for any exhaust leaks

Jack up your car while it’s running and listen and search for any exhaust leaks before the rear O2 sensor. An exhaust leak can make your O2 sensors to make a faulty reading and may cause the P0420 code.

If you have an exhaust leak behind the rear O2 sensor, it should not cause the P0420 code. (If it’s not too close to it).

A tip on how to find exhaust leaks:

6. Test drive

Take the car out for a test drive and make sure there is no misfires or other symptoms. Drive it for some time and drive the car pretty hard to make sure that the catalytic converter is getting really hot. If you have any misfires or other strange behaviors, start to diagnose these first.

7. Drive to your garage and jack it up

Jack up your car and have it at idle. (Make sure that you have good ventilation). Connect your OBD2 scanner and check the live data of both O2 sensors. Open a graph for the signal of the O2 sensors. Rev your car and make sure that the catalytic converter temperature is over 400c. You can use a tool like the Digital Laser Thermometer to check the temperature.

You should have both of the graphs open so you can see them.
The front sensor should jump between 0 – 1 volt all the time and the rear should be stable at 0.7-0.9 if everything is working as expected.

If the catalytic converter is damaged or not hot enough, the rear o2 sensor signal will jump like the front signal.

You can also use a Digital Multimeter for this to check the rear O2 sensor signal, but the signals are very fast and it could be really hard to read the voltage with a multimeter.

8. Check the temperature of the catalytic converter

You can also check the temperature at the catalytic converter. Make sure that your catalytic converter is still hot while checking the temperature. Use a Digital Laser Thermometer to check the temperature.

Check the temperature just before, in the middle and after the catalytic converter. If you get the same temperature at all the points while the catalytic converter is hot ( Over 400c ), the catalytic is most likely empty or damaged.

If the catalytic converter is working properly, you should have a temperature of 100-150c higher directly behind it, than in the front of it. If you have a much higher temperature in the front of the catalytic converter the car may be running rich and the catalytic converter is working too much. It can also be because the catalytic converter is blocked or have a low exhaust flow.

Video of how to check the catalytic converter with a digital laser thermometer:

9. Make a sum of the results

Make a sum of the results from the diagnosing. If you have no other trouble codes, the signal of the read O2 sensor is jumping, the temperature of the catalytic converter is even and you can not see any blue smoke, Replace the catalytic converter.

This is a really hard code to diagnose even for a skilled automotive technician and because the system is so sensitive, it could be hard to diagnose it at the right moment. If you go through this repair guide and you got some faulty results you can replace the catalytic converter with a good confidence.

If everything looks good from the diagnosing but you are still getting the p0420 code, there might still be a bad catalytic converter. The margins can be so small and it can be really tough to see these with any diagnosing tools. You could also have a faulty engine control unit in rare cases.

If you find any misfires, blue smoke or other problems fix these problems and erase the trouble code and if you are lucky the P0420 code may not come back.


What are the possible causes of an internal oil leak?

If you see that your car is smoking blue and you have the P0420 code, as I mentioned, you should repair the oil leak first. But how should you repair it? You may ask. Internal oil leaks can often be very expensive and a full engine job is required sometimes, but in some cases, there could be an easy fault to repair. I will write down the most common causes of an internal oil leak. The best way to find these problems is to do a Leak Down Test to see if there are any pressure losses in the cylinders to check the piston rings.

A clogged Crankcase ventilation system(Most common)

A clogged crankcase ventilation system is the easiest repair to check if you experience blue smoke from your car. You could start by lifting your oil cap when the engine is running. In most cases, the oil cap should be sucked back because most engines have an underpressure if everything is working as it should.

If you experience that you have a lot of pressure in the crankcase, the first thing you should check is the crankcase ventilation. There are often 1-3 hoses going from the crankcase covers to the air filter box. Check these and make sure the hoses are not clogged. A clogged crankcase ventilation can cause the oil to get pushed through the piston rings and the valve seals.

Worn Turbocharger / Supercharger

Another common problem if you have a turbocharged or supercharged car is these are leaking oil, which gets sucked into the engine and makes it combust oil. A simple check for this is to check if your intercooler is filled with oil. If this is the case you may have a worn out turbocharger or supercharger.

You can also remove the boost pipes and check the impeller of the turbocharger to make sure it feels okay and are not loose. It should have a play of ~1mm to the sides, depending on if it’s a ball bearing turbo or not.

Worn Valve Seals

Worn Valve seals can cause oil combustions. However, it’s not a very easy problem to diagnose or repair. You should always check the piston rings with a leak down test before replacing these. To replace the valve seals, the easiest way is to lift the cylinder head and remove the valve springs.

There are tools where you can put pressure inside the cylinder and replace the seals without removing the head, but the procedure is not simple and I do recommend to let a skilled mechanic do it for you.

Worn/damaged Piston rings

Worn or damaged Piston rings are unfortunately a pretty common problem when it comes to internal oil leaks. You can often find it with a Leak Down Test and to inspect the pistons with a small camera through the spark plug holes. If you find out that your piston rings are damaged, the only way is to take the whole engine apart and remove the pistons from the engine block.

If you fit new piston rings, remember to check the pistons and remember to check the clearance gap of the piston rings.

Cracks in engine cylinder head/engine block

Cracks in the cylinder head or the engine block is also a possible cause of oil combustion. However, this problem is not very common, but possible.

If this is the case and you have located the cracks, you should let an expert weld it properly for you if possible. In many cases, you have to replace the whole cylinder head or engine block. You can’t weld it in all situations if you are unlucky.

How does a Catalytic converter work?

Here is a video of how a catalytic converter works:

p0420 catalytic converter

In the picture above, you will see the parts of a catalytic converter. The catalytic converter is used to clean your exhaust gases. If you look into a catalytic converter, you will see that it looks like a honeycomb structure.

From the exhaust gases from a car engine, you will get Hydrocarbons(Unburned fuel), Carbon Monoxide(From engine combustion) and Nitrogen Oxides(created when the heat in the engine forces nitrogen in the air to combine it with oxygen).

The catalytic converter is containing platinum and palladium (That’s why you get paid for a used catalyst). The ceramic structure is converting the carbon monoxide into carbon dioxide when it passes through it. It does also convert hydrocarbons into water and carbon dioxide. It does also convert nitrogen oxides back into oxygen and nitrogen.

If the conversion process fails, the rear O2(Oxygen) sensor will feel it from the signal and the engine control unit will trigger a P0420 code.

How does an O2 sensor work?

Video of how an O2 sensor operates:

What is the function of the front O2 sensor?

The front O2 sensor is a regulation oxygen sensor. It will feel the mixture of the exhaust gases that pass through and it does tell the engine control unit if the mixture is rich or lean.

The Front oxygen sensor can both be mounted directly in front of the catalytic converter or at the exhaust manifold. Some vehicles are using several front O2 sensors for different cylinders. There are two different kinds of O2 sensors – Narrowband sensor ( Most common ) or Wideband sensor (Newer vehicles). The wideband sensor will read the signal a lot faster.

The Narrowband sensor is normally using 4 wires. One power supply, one signal and two heat wires. You can often see if you are having a narrowband or a wideband sensor by looking at the wires. The wideband sensor will have 5 or more wires while the narrowband do have 2-4 wires.

If the front O2 sensor will fail, by it thinks that the engine will run rich when it is running lean, it can trigger the P0420 code.

What is the function of the rear O2 sensor?

The structure of the rear O2 sensor is the same as the front sensor. The difference is that the rear sensor is not giving any information to adjust the mixture to the engine control unit. The rear O2 sensor’s purpose is only to check if the catalytic converter is doing its job properly. If the rear sensor is feeling that the catalytic converter is not working properly, it will send that information to the engine control unit, which will trigger and store a P0420 trouble code.

What is bank 1 vs bank 2?

Bank 1 and Bank 2 are often used if you have more than 4 cylinders. But it is used on 4 cylinder engines also. It means that you do probably have two exhaust pipes or that the engine to separate the cylinders to several O2 sensors. The bank is describing which side/which exhaust pipe the sensor is located at.

Bank 1 is always fitted to the cylinder numbers 1-3-5-7-9 etc while the Bank 2 sensor is fitted to the 2-4-6-8-10 cylinders.

You can unplug one O2 sensor and read the trouble codes with an OBD2 scanner to see what the trouble code is saying. It will most likely tell you if the O2 sensor is located at bank 1 or bank 2.

To find out how to find the right O2 sensor and how to locate different banks, I recommend you to check our article: Bank 1, Bank 2 O2 sensor location.

Can I remove any parts to get rid of the P0420 code?

You can not just remove any parts to fix the P0420 code. It will most likely give you another trouble code or any other symptoms. You can reprogram the engine control unit to take away the catalytic converter monitoring, but it’s not recommended because in the most countries there is a law that you should have your catalytic converter function working.

If you want to reprogram the function you can also remove the catalytic converter completely. Remember that if you remove the catalytic converter you will most likely not pass any emission tests.

There is another way to trick the engine control unit. By replacing the rear oxygen sensor in a pipe. This can work if you desperately want to get rid of the P0420 code but it’s not a recommended method and it may get you into trouble. If you want to check the price and more information about this tool you can check it out here on Amazon: O2 sensor adaptor.

How can combusted oil damage my catalytic converter?

If you have an internal oil leak in your engine which is going through the combustion/cylinder, it will flow out through your exhaust pipe, which creates blue smoke as I mentioned before. What happens if you have an internal oil leak, is that the oil will get stuck and burn in the catalytic converter and damage it.

A catalytic converter can reach over 600 degrees and if it gets too hot, it will get damaged. Then if you just replace the catalytic converter without fixing the oil leak, the oil will continue to get stuck and burned in the catalytic converter and you will damage your new catalytic converter also.

How can a rich or lean mixture trigger the P0420 code?

A rich or lean fuel mixture can damage your catalytic converter in several ways. It can either pour too much fuel into the catalytic converter, which will get ignited in the catalytic converter and damage it. A Lean fuel mixture can cause high exhaust temperatures which can damage the catalytic converter in some situations.

However, the catalytic converter does not get damaged fast because this kind of issues, but in the long run you can wear out the catalytic converter a lot faster than it should if the air-fuel mixture was right.

How can misfires trigger the Po42o code?

Misfires is a common cause for damaged catalytic converters. It is mostly because when a misfire occurs, there will be a fuel that was not ignited which is flowing out through the exhaust manifold into the catalytic converter. Because the catalytic converter is really hot, the fuel will get ignited in the catalytic converter and cause backfires.

These backfires are deadly to any catalytic converter and can damage it really fast. Another way is that the fuel is running out, and directly after there is coming fire from another cylinder, which is igniting the mixture and explodes inside the exhaust pipe. These explosions can damage the catalytic converter really fast.

Recommended tools to fix the P0420 code

To fix the P0420 code you may need some required tools to make the diagnose a lot easier and it will help you to do a good diagnosing.

  • An OBD2 scanner is a required tool to diagnose the trouble code and to check live data and other parameters. I do recommend getting a diagnostic tool that can show the signals in graphs to make the p0420 diagnostics easier.
  • You should always have a Car battery charger while you are doing any work to your vehicle. Low battery voltage can trigger other trouble codes that will confuse you and can cause you to make a faulty diagnosing. Low voltage can also damage control units or other electronics in your car if you are very unlucky.
  • Digital Laser Thermometer is required to check the temperature of your catalytic converter. The temperature value you see with your OBD2 scanner is only a calculated value and it is not the real value. It can also be good for a lot of other tasks when you are troubleshooting your car. Perfect for troubleshooting of your coolant system.
  • Digital Multimeter is required for any electronic measurements and absolutely a tool to invest in. You need a multimeter for almost all troubleshooting in your car electronics and they are not that expensive. Buy a multimeter after your needs, there are both very cheap and expensive multimeters.
  • An O2 sensor adaptor can be used to trick the engine control unit and it may fix your P0420 code. This is not a recommended method as it may break any laws, depending on which county you live in.
  • If you think your catalytic converter is dirty or have oil in it from earlier internal oil leaks, you can try to use a Catalytic converter cleaner to clean it. It’s also used for other tasks to clean your exhaust emissions.


Known P0420 Causes by Car Models

The P0420 code is more known in some car models than others. Here is a list of the most common causes per car brand. These car models are known to having a problem with the P0420 code. Remember that this is only general guidelines, and you should be doing a proper diagnose before replacing any parts.

Toyota Corolla

The most common cause when you find the P0420 on the Toyota Corolla is actually a bad catalytic converter. But, this can often be caused by oil going through the piston rings to get stuck on the catalytic converter. If you have a Toyota Corolla that is struggling with the P0420 code. Check for vacuum leaks and exhaust leaks first. Then check if you can notice any blue smoke from the exhaust pipe. If you can find it, it’s a sign that you might want to get expert help to find out where the oil is coming from. A standard thing can be to check the crankcase ventilation.

If you do not notice any blue smoke at any RPM. There is most likely your catalytic converter that is worn out.

Ford Focus

Ford focus does normally have vacuum leaks or any broken solenoid which is causing faulty air-fuel mixture and then causing the P0420 code. Check your trouble code memory with a diagnostic scanner to see if you can find any trouble codes about the air-fuel mixture. If everything looks fine, check for exhaust leaks. Replace catalytic converter if you can’t find any trouble codes or other problems with the air-fuel mixture.

Subaru / Subaru Forester

Subaru does normally have the same problem that the Toyota corollas have. Check for vacuum leaks or other fuel mixture related trouble codes. Check for any exhaust leaks before the catalytic converter. The most common problem on Subaru engines is actually the catalytic converter itself.

Volkswagen (VW) / Skoda / Seat / Audi A4 1.8T / V6 2.4

These VAG cars do have some known problems causing the P0420 code. Check the function of the check valves under the intake and make sure the crankcase ventilation is free from dirt, causing the engine to burn oil which makes the catalytic converter clogged. Check for exhaust leaks around any flex pipes on the exhaust pipe (Common cause). Check for any trouble codes of the O2 sensors. If no problem is found, replace the catalytic converter. It’s a very common problem on both the 1.8T and the V6 petrol engines. The 1.8T catalytic converter can be pretty difficult to replace if you do not have a good experience. The V6 have two catalytic converters, so make sure that you troubleshoot and replace the catalytic converter on the right bank.



  • In the most cases, the catalytic converter is faulty and the cause to the P0420 code. But there can be another reason behind why the catalytic converter was getting damaged. Always fix all other trouble codes before you are fixing the P0420 code.
  • You can use special tools or reprogram the engine control unit to trick the control unit to get rid of the P0420 code
  • Bank 1, Bank 2 is pointing to which of the O2 sensors that are faulty or which catalytic converter that is faulty.
  • The catalytic converter is cleaning the exhaust emissions and may not be removed by law.

If you have any other questions about the P0420 code or related questions, you can comment down below and I will answer your questions as fast as possible. If you have any other car related questions you are welcome to ask them on our homepage.

To find all the OBD2 codes. Check our OBD2 Code List.

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