Register Now
Lost Password

Lost your password? Please enter your email address. You will receive a link and will create a new password via email.

Register Now

Welcome to Ask A Master Mechanic. Please register to get fast help with your vehicle for free!

1 Star2 Stars3 Stars4 Stars5 Stars (1 votes, average: 5.00 out of 5)

P0420 Code Fix: The Complete Guide


In this article, you will find everything you have to know about the P0420 code to finally get rid of it once for all.

We are getting a lot of messages from our customers of the P0420 code. It keep’s 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. I’m working as a diagnostic technician and I have fixed these problems a lot of times. I will share my knowledge with you about how to get rid of it once for all.

It is not really that hard if you get the right information about the code.


In this article:


What does the P0420 code mean?

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.

If the rear O2 sensor is getting faulty values, the p0420 code will be set. This could be because the catalytic converter is faulty, or a false alarm because one of the O2 sensors is damaged. There can 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.


Possible 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.

  • Check engine light
  • Misfires
  • Rich Mixture
  • Lean Mixture
  • Motor Oil Burn/Blue smoke from the exhaust pipe
  • Slow acceleration


Possible 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)


Possible 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
  • Replace Engine control unit (Rare)


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 you car is not burning any oil by checking the exhaust smoke, Blue Smoke = Oil, White = Water, Gray/Black = Rich Mixture.

Examples of things that could damage the catalytic converter and O2 sensors

  • Misfires
  • Air Leak
  • Lean fuel mixture
  • Rich fuel mixture
  • Ignition Timing
  • Oil burning ( Oil gets into the catalytic converter and O2 sensors )
  • Leaking fuel injectors
  • Wrong kind of fuel
  • Faulty sensors ( Coolant temperature, air temperature for example )


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.


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).


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 behaviours, 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 ThermometerP0420 Code Fix: The Complete Guide 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 MultimeterP0420 Code Fix: The Complete Guide 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 ThermometerP0420 Code Fix: The Complete Guide 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:

10. 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.


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 diagnosing 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 MultimeterP0420 Code Fix: The Complete Guide 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 adaptorP0420 Code Fix: The Complete Guide 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 cleanerP0420 Code Fix: The Complete Guide to clean it. It’s also used for other tasks to clean your exhaust emissions.


How does a Catalytic converter work?

p0420 catalytic converter

In this picture, 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.

P0420 Code Fix

In this picture, you will see the function of the catalytic converter monitoring system. A Lambda sensor is the same as an O2 oxygen sensor(Just different names).


What is the function of the front O2 sensor?

P0420 Code Fix: The Complete Guide

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



  • 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.

Leave a reply

Ask A Mechanic Admin

About Ask A Mechanic AdminDiagnostic Technician

Hello. I live in sweden and work as an Automotive Diagnostic Technician. Worked with the most brands in the most advanced fault diagnostics. Im here to answer all your automotive questions. If you have any private questions you could send me a PM and I'll answer them as soon as possible!

Follow Me