P0420 Code – Meaning, Causes, Symptoms (Simple Fixes)

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

P0420 Trouble Code

P0420 is a trouble code that is stored in your car engine control module when it recognizes a problem with the catalytic converter’s efficiency.

This means that something in your car’s emissions system has failed and probably needs to be replaced. Not only does this pose a problem with the environment, but it can also put you at risk of running into mechanical issues down the road if not properly taken care of.

Keep reading to find out what causes this issue, how to fix it yourself, and what you should do if you cannot resolve it yourself!


P0420 – Catalyst System Efficiency Below Threshold (Bank 1)

What does the P0420 Code Mean?

Code P0420 indicates the catalytic converter’s efficiency is below the threshold. It can either be caused by a faulty catalytic converter or a false alarm from any of the sensors.

The ECM uses two O2 sensors, one in the front and one in the back of the catalytic converter, to measure efficiency. If the catalyst system efficiency is low, the P0420 code will be triggered. In most cases, a P0420 trouble code is caused by a bad catalytic converter.

P0420 Symptoms

The most common symptom of code P0420 is a check engine light on the dashboard. In rare cases, you can also notice issues like misfires, higher fuel consumption, or a bad odor smell coming from the exhaust pipe.

However, In most cases, you will most likely notice any symptoms except the check engine light with the P0420 code stored in the engine control unit.

How serious is the P0420 code?

Low – The P0420 code will in most cases not create any further problems with your car’s engine.

The only thing that can happen is that the catalytic converter is so damaged that the catalytic converter parts come loose and blocks the exhaust pipe, which is pretty unlikely to happen.

The P0420 trouble code makes your car’s emissions bad for the environment, though, so you should fix it as soon as possible.

What Is the Causes of P0420?

The most common cause of the P0420 code is a faulty catalytic converter. It can also be caused by a faulty oxygen sensor, exhaust leak, intake leak, or wrong placement of the catalytic converter.

It could be a faulty bad catalytic converter, but I have seen many cases when the catalytic converter is pretty new, but it’s not an original OEM part. Some cheaper catalytic converters may not be enough, and in some cases, you have to buy a brand new catalytic converter from the manufacturer.

I have also seen non-OEM catalytic converters that have been installed too far away from the engine on the exhaust pipe. Because of this, the catalytic converter will not get hot enough and will trigger a P0420 trouble code. Here are all possible causes of P0420:

  • Damaged catalytic converter (most common)
  • Not Genuine catalytic converter
  • Wrong placement 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)
  • A faulty engine control unit (rare)

What repairs can fix the P0420 code?

You should first check the function of the catalytic converter with the methods further down in the article before replacing any parts. You will most likely waste your money by just replacing parts, so it is much better to make a proper diagnosis. However, here are some things that can fix the P0420 code:

  • Cleaning the catalytic converter
  • Replace catalytic converter
  • Replace to a genuine original catalytic converter
  • Replace front oxygen sensor
  • Replace rear oxygen sensor
  • Repair faulty wirings
  • Fix oil burn
  • Fix misfires
  • Fix lean / rich fuel mixture
  • Check the data with an OBD2 scanner
  • Replace engine control unit (rare)

Common P0420 Diagnosis mistakes

The most common mistake is to replace the oxygen sensors without doing any proper diagnostics. The cause of this trouble code is most often the catalytic converter – which may get damaged by other problems with your car’s engine like misfires.

Bad O2 Sensors can cause this trouble code but is pretty rare.

Common P0420 Code Causes by Car Model

The P0420 trouble 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 this trouble code

Remember that these are only general guidelines, and you should be making a proper diagnosis before replacing any parts.

1. Toyota Corolla

The most common cause when you find this trouble code on the Toyota Corolla is a bad catalytic converter. 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 is worn out.

2. 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.

3. 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 with Subaru engines is the catalytic converter itself.

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

These VAG cars have some known problems causing the P0420 trouble code. Check the check valves’ function 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 oxygen sensors. If no problems were found, replace the catalytic converter. It’s a widespread 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, ensuring you troubleshoot and replace the catalytic converter on the right bank.

How to Diagnose code P0420

P0420 Check Engine Light

The P0420 Code is mostly caused by a faulty catalytic converter as mentioned before. You should always diagnose it properly with the methods down below before replacing anything.

However, you can clean the catalytic converter by using an additive in the fuel tank in some cases. There are many different additives on the market, so we recommend choosing one of the best catalytic cleaners from our list.

  1. Connect an OBD2 Scanner and look for related trouble codes. Repair any related trouble codes regarding ignition or fuel before you try to fix the code.
  2. Check the live data to see the front and read O2 sensor signals. The car engine should be scorching – and the front sensor should fluctuate between 0-1 volts, and the rear should be steady at 0.7 – 0.9 volt. If it is not, there is a risk that the catalytic converter is defective.
  3. Heat the engine and check the temperature at the front of the catalytic converter and then in the rear. If the engine is hot and there is no difference in temperature before and after the catalytic converter – your catalytic converter does probably not work.
  4. If the catalytic converter is installed easily, it can be worth removing the pipe from one end of it and check inside the catalytic converter for any visual damages.
  5. If everything points at a faulty catalytic converter – replace it. If you can’t find any problem with the temperature, voltage, or a visual inspection, you should try to repair other related trouble codes and then clear the codes and try again.
  6. If you still can’t find any problems. Make sure it is a genuine OEM catalytic converter, and it is installed in the original place. If everything seems fine – replace the catalytic converter.

Check out this video for more advanced P0420 Diagnosis.

Catalytic Converter Damaged Causes

There are a few things that are known for damaging the catalytic converter, which can cause the trouble 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. Ensure that your car is not burning any oil by checking the exhaust smoke, Blue Smoke = Oil, White = Water, Gray/Black = Rich Mixture.

Estimated Repair Cost

The estimated cost of repairing the code is the following. The prices are including parts and labor work at a workshop. The costs do not include diagnosis costs.

  • Catalytic Converter Replacement – 500$ to 1500$
  • Front Oxygen Sensor Replacement – 150$ to 300$
  • Rear Oxygen Sensor Replacement – 150$ to 300$

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

You cannot just remove any parts to fix this trouble 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. Still, 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. Remember to check the laws in your state or country before installing one of these.

Common P0420 Related Questions

How to fix the P0420 code?

To fix the P0420 code, you need to diagnose what is causing the trouble code. Start with inspecting and diagnosing your catalytic converter and continue with checking the O2 sensors.

What can cause a P0420 code?

A bad catalytic converter is the most common cause of the p0420 code. This doesn’t mean that you should replace it, though. Always do proper research before replacing parts to save money.

What does code P0420 Bank 1 mean?

The P0420 trouble code means that rear O2 sensors report to the engine control module that the catalytic converter is not doing it’s job properly. The rear O2 sensor is comparing the signal from the front O2 sensor.

How to clear code P0420?

To clear the P0420 code you need to use an OBD2 scanner. Remember that just clearing the P0420 code will most likely solve the problem, you do also have to fix the problem.

How much does it cost to fix a P0420 code?

There is no fixed price to fix the P0420 code. However, it is often caused by a faulty catalytic converter, and one of these usually costs 500$ to 1000$ for the part and 100$ – 200$ in replacement cost.

Can I drive with the P0420 code?

The P0420 code itself is not causing any serious damages to your vehicle for short distances. However, driving long distances and ignoring the trouble code is not recommended. Fix it as soon as possible.

Need a repair manual?

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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.