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

A guide on how to diagnose & repair problems causing the P0420 code

P0420 Check Engine Light

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

There are many different reasons for this, so you need to know how to diagnose this trouble code properly.

Code P0420 Definition

Catalyst System Efficiency Below Threshold (Bank 1)

What does the P0420 Code Mean?

A P0420 Code means that the engine control unit recognizes that the catalytic converter’s efficiency 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. In most cases, a P0420 code is caused by a bad catalytic converter.

P0420 Code Symptoms

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

  • Check engine light
  • Misfires
  • Rich fuel mixture
  • Lean fuel mixture
  • Bad Odour Smell

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

What causes the P0420 Code?

The most common cause of the P0420 code is a faulty 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 an 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 code.

  • 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. By just replacing parts, you will most likely waste your money, so it is much better to make a proper diagnose. However, here are some things that can fix the the code:

  • Cleaning the 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 fuel mixture
  • Check the data with an OBD2 scanner
  • Replace engine control unit (rare)

Common Diagnosis mistakes

The most common mistake is to replace the O2 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 the this trouble code, but is pretty rare.

Causes by Car Models

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 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 O2 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 the P0420 Code

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 O2 Sensor Replacement – 150$ to 300$
  • Rear O2 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 Related Questions

How to fix 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 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 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.

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.