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P0420 Code Fix DTC: Catalyst System Efficiency Below Threshold (Bank 1)

Short Diagnostics ( In-depth diagnostic guide lower down in the article ):

Possible Causes/Solutions:

  • Damaged Catalytic converter ( Most common )
  • Damaged upstream front O2 Sensor / Faulty wirings
  • Damaged downstream rear O2 Sensor / Faulty wirings
  • Exhaust Leak


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 will be damaged again. Check your DTC memory to find a combination of any other trouble codes Fix them before the P0420 Code. Check that the car is not smoking blue, that is because the engine is consuming engine oil and it will damage your exhaust parts.

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 sensor ( Coolant temperature, air temperature for example )



In-depth diagnostics guide:


Is your car suffering from the trouble code p0420 – Catalyst System Efficiency Below Threshold (Bank 1) or P0430 (bank2) over and over again?

My name is Magnus and I’m working as a diagnostic technician. I’m searching faults on vehicles all days long and got some good experience of how to find the faults fast and with a good result. I am sharing this article with you to help you to solve this trouble code fast and saving you some money. The P0420 trouble code is a very common trouble code that I have dealt with a lot of times, and for sure it can really be a pain in the *** code if you don’t know where to start.

The most common thing is that you think you have solved the fault, and then after you drove around 100 km’s ( 60 miles) suddenly the engine lights come back on again. Back to the workshop and throw out more money to your mechanic that’s not have an idea of what he is doing, he is just replacing parts one after one and sucking out your money. But it’s not hard to find and solve the fault for this trouble code if you know where to look. That’s what this article is going to be about. How to fix this fast and cheap by doing it yourself at home.


In this guide:

  1. The fast and easy solution
  2. My engine is running fine with the engine light ON, why do I have to fix this?
  3. Can I drive with the P0420 and Engine MIL on?
  4. Can I remove the catalytic converter or sensors?
  5. How to recognise bank 1 vs bank 2
  6. How to diagnose the p0420 code
  7. Summary

The fast and easy answer:

I guess that most of you want a fast and reliable answer to this question and here is the fast answer. Remember that this list may not solve your problem. If you want to be 100% sure then you should read the whole article.  This list is just based on my own experience from the most common solutions for this trouble code Po420.

  1. Faulty catalytic converter, sometimes caused by an internal oil leak inside the engine that is destroying the catalytic converter.
  2. Have to be fixed first
  3. Not a genuine OEM catalytic converter. Aftermarket catalytic converters can often give your this p0420 code because the function is not the same as the OEM part.
  4. Exhaust leak
  5. Faulty upstream or downstream 02 Lambda sensors.


NOTE: Also, If your engine control unit has other engine related DTC trouble codes like P0300, P0301 etc, ALWAYS FIX THEM FIRST before you try to fix the P0420 code.

If your car is misfiring it will damage your catalytic converter and if you replace your catalytic converter, you will destroy the catalytic converter again. It is a very expensive part so you really want to make sure to fix all misfires, lean mixtures and internal oil burning first. Fix all the other trouble codes first to be sure. Because this trouble code can come back intermittently after a lot of driving and if you don’t want to go to a car workshop every time. I can recommend you to buy a DTC diagnostic tool to use at home. “Aren’t those tools very expensive?” you may think. The answer to that is no, there is a bunch of cheap tools out there for an affordable price if you are only going to read the generic engine OBDII memory.

If you are interested in buying one, you can check our review of the Best OBD2 Scanners.

If you are interested to buy one of this tools I can recommend this tool from It’s a good tool for a reliable price that is working with the most car models. But always read the description on Amazon to make sure it works with your vehicle before you are buying it. Now I have written the fast answer for this trouble code, but there are still much more to learn about this subject if you want to make sure that you fix this p0420 trouble code once for all. If you have more questions or are interested to learn more deeply how to diagnose this trouble code, just continue reading.



My engine is running fine with the engine light ON, why do I have to fix this?

The P0420 code does never make your engine run badly. In that case, if it’s running roughly there is probably another fault in the vehicle and you will probably have another trouble code stored in the DTC memory. What this P0420 code is telling you, is that the cleaning function of the catalytic converter is not working as it should. What it’s telling us could be true or false. There could be a faulty catalytic converter, but it can also be anything that makes the ECU think that there is a fault with the catalytic converter but it is not in reality. You can also have a fault in the engine that made damage to the catalytic converter.

P0420 Code Fix

In this beautiful picture (that I made myself in paint) is explaining the function of a catalytic converter. The O2 Oxygen (lambda) sensors are feeling the difference between both sides of the catalytic converter. The oxygen 02 sensors are measuring the amount of oxygen in the exhaust (obvious maybe). When the Lambda 02 sensor 2 tells the ECM that the catalytic converter is not doing the job properly, then the fault code P0420 is showing up in your DTC trouble memory.

The front upstream 02 oxygen sensor is regulating the air mixture. The rear downstream 02 oxygen sensor is a diagnostic sensor. It won’t regulate the air mixture, only check the function of the catalytic converter all the time. As you see, if there is an exhaust leak it could trick one of the oxygen 02 sensors and therefore it thinks that the catalytic converter is not working properly. There could also be a damaged 02 oxygen sensor, and in that case, both 02 sensors are not matching and that will result in a trouble code trigger.




p0420 catalytic converter

Here is a picture that is showing you a bit more of what is inside a catalytic converter if you are interested in the chemical designations. If you are looking into a catalytic converter it really looks like it’s a home for some bees that are saving their honey there. Because there are so many chemical substances and even noble metals inside, you maybe understand why this piece is not so cheap. People are even coming to my workshop several times a week and asking if they can buy used and damaged catalytic converters from me because they can sell it at a good price. There was a rumour that it was a time when you could almost buy new pirate/not genuine catalytic converters and sell them for more at car graveyards. I don’t know if it’s true really, but it’s not impossible if you are looking at the prices they are paying for used catalytic converters



Can I drive with the P0420 and Engine MIL on?


p0420 code fix

You can drive with your MIL on and the P0420 trouble code in the memory if you want because the trouble code is not making the engine to run bad itself. But I don’t recommend that you are driving it for a long distance because when your engine light is on, your mixture regulation and other emissions may not function properly. It could cause other faults on the engine in the long term. But if you are driving for kinda short distances to your workshop or down to the market it won’t hurt, if you are not having any other issues with the engine that is causing the p0420 code. Another negative effect is that if you are driving with the engine light on, you won’t notice if there are other trouble codes that are coming up. It could be trouble codes that really can hurt the engine if you are still driving it and it will be hard to notice with the engine light on.

So my short answer to this is: You can drive for short distances without problems because the trouble code itself won’t damage your engine, but you should fix the problem as soon as possible. Another thing to keep in mind is if your catalytic converter is damaged inside, a piece inside the cat could come loose and lay on the cross and blocking the exhaust flow. In the worst case, it will block all the exhaust flow and your engine won’t run.


I don’t want to pay for a new catalytic converter, can I remove the catalytic converter or sensors?


pay p0420

No, you can’t remove any parts to get rid of this easily. If you are removing the catalytic converter or the rear lambda sensor you will get the same trouble code and other, new codes registered. The engine control unit is programmed with this function for the environment and if you are removing any of these parts, the engine control unit will see it. What you can do is to reprogram your engine control module by deleting the catalytic monitoring function inside the code. There are several tuning shops that can do this for you. But they won’t do it for free and you should only do it if you are going to use the ECM for motorsports. Remember that deleting the catalytic function is against the rules and in the most countries, you could get a huge fine for this if you are destroying the environment to saving some bucks.

Another thing that I have seen, but not tried myself is to put the rear 02 oxygen sensor inside a short tube to trick the engine control unit. I have not tried this fix myself so I cant tell you if it will work or not, but I have heard from several people that are saying that it fixed the p0420 code for them. The pieces are not very expensive and could be worth a try. Remember to only use this for motorsports, where it’s legal.

Here are some examples of what you need:

If you want to save some money you can buy a used catalytic converter from a car that has low mileage. I don’t recommend to buy a catalytic converter from a car that has high mileage because it may give you the same trouble code again, especially if it has been laying outside in the car-graveyard for a long time and after that, it could be hard to recover it. A tip is to check on eBay or google for used catalytic converters, there is a lot of good information and sales out there if you know how to use it.

My DTC fault memory gives me both P0420 (bank 1) and P0430 (bank 2), Whats the difference?


The most of the cars only have one “diagnostic 02 sensors”, the rear of them. But some v6/v8/v10 and other big engines have two catalytic converters and four 02 oxygen sensors.
This makes the diagnostic process harder because now you have two catalytic sensors to search problems on and it could be hard to identify which side is which. If you have both P0420 and P0430 then you should go by the same diagnostic principle. It could be either that both catalytic converters are worn out, or something inside the engine that destroyed both of them (oil burning or misfires as an example). It could also be an air leak or a faulty rear oxygen sensor.

Check our guide Bank 1 vs Bank 2 O2 sensor location to learn more.

A tip to identify which side is bank 1 and which is bank 2 if you have a diagnostic tool is to clear the DTC code memory. After that, you unplug a 02 oxygen sensor on that side and check the DTC memory. You will now have a new trouble code on the 02 sensors and it will probably tell you if the sensor is on bank 1 or 2. If you are not getting a trouble code after you unplugged the 02 sensors you can go to live data and check which of the sensors have 0 volts when the ignitions are on.

You should also know that some engines have two front sensors, one catalytic converter and one rear diagnostic 02 sensor.

I want to fix this problem and diagnose it properly. How do I do it?

p0420 fix

If you want to check your catalytic converter, the easiest thing is to take it out for a long drive to get the catalytic converter very hot. After your long drive, you can drive it inside your garage and jack it up fast and at the same time as the car is running (Use good ventilation) you should measure the temperature before and after the catalytic converter. To measure the temperature easily, I can recommend using a tool like the Etekcity 1022D Dual Laser Digital Infrared Thermometer Link to Amazon.

If you get the same temperature before and after the catalytic converter when it’s hot then the function of the catalytic converter is not working properly and it may be nothing left inside the catalytic converter.

Another tip of how to diagnose this problem is to check the voltages of the 02 sensors. The easiest way to do this is to use a diagnostic tool. Use a good diagnostic tool that can measure the live data. To find a good OBD2 scanner you should check out our article OBD2 scanner reviews.

You can also measure the voltage with a multimeter but it’s much easier with a diagnostic tool. What you should check is the live data of the voltage of the 02 sensors. Also here you have to drive the car really hot first and then you should look at the voltage at a diagram. The front oxygen sensor voltage will jump between 0-1 volt all the time is the function is okay. The rear sensor should not do that if it’s working properly. It should have a stable voltage at around 0.6-0.8 volt. If the voltage of both sensors is going at the same line your catalytic converter is probably worn out.

I do not recommend this method because it is probably going to work when you are revving the engine inside the garage and it’s hard to get the catalytic converter hot enough. It could lead to a false positive test. So in the most cases when I’m doing this is to use the temperature meters. It’s a lot of text for you right now and because it’s in the most cases easier to watch a video. I would recommend you to watch this video to learn more about this diagnostic method that I was explaining to you.


  • The most common fault is actually a faulty Catalytic converter
  • It could be other faults in the engine that have destroyed the catalytic converter
  • Always fix other engine related fault codes before the p0420 DTC code
  • You should not run the engine for too long with the MIL ON/P0420 code registered in the DTC trouble code memory

I hope that you found what you were seeking for in this article. I think that I explained everything you have to know, to get rid of this trouble code without throwing all your money away. Now you can go to your local mechanic and tell him how to fix it 😉 From the beginning when I was starting to write this article, I was going to explain fast in short text of how to fix this fault code. But in the end, it almost became a book with over 2500 words. I hope that I have answered your questions about how to get rid of this trouble code. There is so much to read and learn about this fault code so I don’t think I can explain it good enough.

If you have more questions or want me to add anything to this article, then post a comment down below here or you can make a question on our Ask A Question page! Make a good and detailed question and you will get fast and professional help from one of our diagnostic technicians/master technicians as fast as possible. Good luck and I hope to hear more from you about your diagnostic results after reading this article. If you like the article, remember to share it with your friends! See you in the next article!

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