Are you struggling to get rid of your P0420 code?

Then this is the article for you. The P0420 code can be a bit tricky to diagnose correctly, as you think that you have fixed the fault and then 100 km later, the light pops up again.

This code is triggered when the engine control unit recognizes 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 to measure the efficiency of the catalytic converter.

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

P0420 Code Meaning?

P0420 means that the efficiency of the catalytic converter is below the threshold. The ECM uses two O2 sensors, one in the front and one in the back of the catalytic converter to measure the efficiency. If the efficiency is low, the P0420 code will be triggered.

We receive 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. This P0420 trouble code may confuse a lot of people because there is much false information about it out there.

The front O2 sensor measures the mixture, and it tells the engine control unit if the engine is running with a rich or lean mixture and adjusts it.

The rear O2 sensor is just a diagnostic O2 sensor for this p0420 trouble code.

If the rear O2 sensor is getting faulty values, the p0420 code will get stored in the engine control unit. It could be because the catalytic converter is defective, 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 install a new catalytic converter, it may be damaged again.

A lot of people think that lean or rich mixtures could trigger the trouble code. Yes, they 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 show the trigger the code Po420.

Possible Symptoms

Check engine light

Misfires

Rich mixture

Lean mixture

Motor Oil burn/blue smoke from the exhaust pipe

Slow acceleration

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

Possible Causes

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)

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 an excellent 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.

Possible Solutions

These are just possible solutions, and you should never replace parts without doing proper diagnostics; 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 this p0420 trouble code properly in the same way as a skilled automotive technician would do. 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 Diagnosis Table

Code Symptoms Causes Cat Damaging causes P0420 Check engine light



Misfires



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 Misfires



Oil Consumption



Exhaust Leak



Intake leak



Rich Mixture



Lean Mixture



Bad ECM/PCM



Catalytic Converter Damaged Causes

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

Misfires

Oil consumption

Exhaust leak

Intake leak

Rich mixture

Lean mixture

Bad ECM/PCM[/su_tab]

Important Note:

Many things could cause the O2 sensors or the catalytic converter to fail. You have to make sure that you fix these problems before you replace 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 this trouble 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

Here is a guide to how a professional automotive technician would diagnose the p0420 trouble code. This guide may require some car electrical skills and you may need some tools for the task. But 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 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. A 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 perform any basic settings, reset adaptions and other functions that are required to fulfill the diagnosis. Check the link above to see several different OBD2 scanners and learn the pros and cons of some different OBD2 scanners.

Most OBD2 scanners can only read the engine control unit. If you want to read the trouble codes of 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 also fix the P0420 code. Fix all the other trouble codes before trying to fix the P0420 code.

Always note down any trouble code numbers and 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 stop them from reading the mixture properly.

You should always check for white, grey and black smoke. White smoke comes from water in the exhaust pipe and grey/ black smoke comes 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 lead to your O2 sensors giving a faulty reading and may cause the code to trigger.

If you have an exhaust leak behind the rear O2 sensor, it should not cause the 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 are 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 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 proper 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 volts 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 high-speed and it could be tough 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 monitoring 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 correctly, 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 has 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 summary of the results from the diagnositics. 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.

The P0420 code can be a problematic trouble code to diagnose correctly. If you go through this repair guide and you get some incorrect results, you can replace the catalytic converter with reasonable confidence.

If everything looks good from the diagnostics, but you are still getting the p0420 code, there is a small chance that the catalytic converter is still the problem causing the trouble code. 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 p0420 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 fix it? You may ask. Internal oil leaks can often be costly and a full engine job is required sometimes, but in some cases, there could be a natural 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.

1. A Clogged Crankcase Ventilation System (Most Common)

A clogged crankcase ventilation system is the most natural 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 suck back because most car engines have an underpressure if everything is working as it should.

If you experience 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 tubes are not clogged. Clogged crankcase ventilation can cause the oil to get pushed through the piston rings and the valve seals.

2. 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 fills up with engine 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 is not loose. It should have a play of ~1mm to the sides, depending on if it’s a ball bearing turbo or not.

3. 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. The easiest way to replace the valve seals 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 some situations though.

How Does a Catalytic Converter Work?

Here is a video of how a catalytic converter works:

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 contains platinum and palladium (that’s why you get paid for a used catalyst). The ceramic structure converts the carbon monoxide into carbon dioxide when it passes through it. It also converts hydrocarbons into water and carbon dioxide. Additionally, it converts 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 sense the mixture of the exhaust gases that pass through, and it does tell the engine control unit if the mix 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 usually uses four wires. One power supply, one signal, and two heat wires. You can often see if you have a narrowband or a wideband sensor by looking at the cables. The wideband sensor will have 5 or more wires while the narrowband has 2-4 wires.

If the front O2 sensor fails becuase 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 correctly. If the rear sensor picks up that the catalytic converter is not working correctly, 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 typically used if you have more than four cylinders. But it is used on four-cylinder engines also. It means that you probably have two exhaust pipes or that the engine to separate the cylinders is linked to several O2 sensors. The bank is describing at which side/which exhaust pipe the sensor is located.

Bank 1 is always fitted to the cylinder numbers 1-3-5-7-9 etc. while the Bank 2 sensor is provided 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 bank., 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 cannot just remove any parts to fix the P0420 code. It will most likely give you another trouble code or some other symptoms. You can reprogram the engine control unit to take away the catalytic converter monitoring, but it’s not recommended because in 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 delete the catalytic converter. 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 method 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’re going 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 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 also damage your new catalytic converter.

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 ruin 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 of these kinds of issues, but in the long run you can wear out the catalytic converter a lot more quickly than it should if the air-fuel mixture was right.

How Can Misfires Trigger the P0420 Code?

Misfires are a common cause for damaged catalytic converters. It is because when a misfire occurs, there will be a fuel that is not getting ignited which is flowing out through the exhaust manifold into the catalytic converter. Because the catalytic converter is 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 fast. Another way is that the fuel is running out, and directly after, fire comes from another cylinder, which ignites the mixture and explodes inside the exhaust pipe. These explosions can damage the catalytic converter fast.

To fix the p0420 trouble code, you may need some required tools to make the diagnosis a lot easier and it will help you to carry out a good diagnostic test.

An OBD2 scanner is a required tool to diagnose the trouble code and to check live data and other parameters. I recommend getting a diagnostic tool that can show the signals in graphs to make the diagnostics easier.

You should always have a car battery charger while you are doing any work on your vehicle. Low battery voltage can trigger other trouble codes that will confuse you and can cause you to make a faulty diagnosis. Low voltage can also damage control units or other electronics in your car if you are very unlucky.

A 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.

A migital 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 according to your needs; there are both very cheap and costly 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 some laws, depending on which county you live in.

If you think your catalytic converter is dirty or has 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 common in some car models than others. Here is a list of the most common causes per car brand. These car models are known to have a problem with the P0420 code. Remember that these are only general guidelines, and you should be doing a proper diagnosis before replacing any parts.

Toyota Corolla

The most common cause when you find the P0420 on the Toyota Corolla is 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 trouble code. Check for vacuum leaks and exhaust leaks first. Then check to see if you notice any blue smoke coming from the exhaust pipe. If so, it’s a sign that you might want to get expert help to find out where the oil is coming from. A standard check is to check the crankcase ventilation.

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

Ford Focus

The Ford Focus usually has vacuum leaks or any broken solenoid which causes a faulty air-fuel mixture and then causes the trouble 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 the catalytic converter if you can’t find any trouble codes or other problems with the air-fuel mixture.

Subaru / Subaru Forester

The Subaru usually has 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 the catalytic converter itself.

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

These VAG cars 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 clogs the catalytic converter. 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 were 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 lot of experience. The V6 has two catalytic converters, so make sure that you troubleshoot and replace the catalytic converter on the right bank.

How do you clear the P0420 Code? To clear the P0420 code you should use an OBD2 scanner. Remember that clearing this trouble code will most often not solve the problem. The problem still occurs and the trouble code will come back sooner or later. What is the most common cause of P0420? Actually a bad catalytic converter is the most common cause of the p0420 code. This doesn’t mean that you should just replace it though. Always make proper research before replacing parts to save money. Can you test your catalytic converter for the P0420? Yes, there are several ways to test your P0420 code, and these how you do it, you will find an answer about in this article.

Conclusion

In 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 fix the trouble 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 cleans the exhaust emissions and may not be removed by law.

If you have any other questions about the trouble code or related issues, you can comment 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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P0420 Code – Meaning, Symptoms, Causes & Fixes was last modified: by