The P0420 code - Definition, Causes, and Symptoms (& How to Fix It)

Because it is difficult to remove, P0420 code is well-known in the automobile industry. This article will provide information on what the error code P0420 is, how it can be fixed, and why it might occur.

It can be difficult to remove P0420 without proper knowledge. We have seen it happen before.

In this blog post, we’ll answer all these questions and more. You can read on to find out everything about the code P0420.

Definition of Code

P0420 – Catalyst System Efficiency Below Threshold (Bank 1)

What does the P0420 Cod mean?

The P0420 code indicates that the ECM has detected underperformance of the catalytic converter’s efficiency on Bank 1. This can be due to a defective catalyst, or false alarms caused by an O2 sensor.

Two O2 sensors are used by the engine control module (ECM), one at the front and one behind a catalytic converter, to determine its efficiency. A P0420 error code is issued if there are low catalyst system efficiency. A bad catalytic convert is usually responsible for triggering the P0420 code.

An incorrect air-fuel mixture can also temporarily or permanently reduce the catalyst’s efficiency, resulting in a PO420 code.

Is there a reason for the P0420? 

A faulty catalytic convert is the most frequent cause of P0420 codes. You can also get it from a defective oxygen sensor, an intake leak or exhaust system leak.

This could mean that the catalyst has a defect. However, it is possible for it to be an OEM part. Cheaper catalysts might not work well enough. You will likely need to order a brand-new catalyst from the manufacturer.

There have been non-OEM converters installed far too close to an engine's exhaust pipe, towards the last muffler. This will cause the catalytic converter to not heat up enough, and will result in the trouble code P0420.

If the car’s air-fuel mixture is incorrect due to a faulty injector, high fuel pressure, faulty fuel system, or a damaged sensor, it may cause the catalytic converter to not function properly.

The following are some of the causes for P0420:

• Catalytic converter damage (most common)
• The catalytic convert is prone to oil contamination
• Improper placement of the catalytic convert
• Faulty wiring / damaged oxygen sensor upstream and downstream
• Lunge in exhaust manifold, or from the exhaust system
• Wrong type of fuel (leaded fuel instead of unleaded)
• A mixture of lean and rich (damaging the catalytic convertor)
• Misfires can cause damage to the catalytic convert
• Faulty engine control module (rare).

Which repairs are possible to fix the code P0420

Before you replace any part, make sure to first verify the functioning of your catalytic converter using the techniques further down. It is likely that you will waste money replacing the parts. Therefore, it is better to do a complete diagnosis. Here are some ways to fix error code P0420

• Clean the catalytic convertor
• Change catalytic converter
• Replace front oxygen sensor or rear oxygen sensor
• Repair damaged sensor wiring
• Replacing damaged engine sensors
• Fix high oil consumption
• Fix misfires
• Mix lean and rich fuels
• Replace engine control unit (rare)

What are the symptoms of a code P0420? 

The most common symptom of code P0420 is a check engine light on your car’s dashboard. Rarely, other issues may be present such as misfires, increased fuel consumption or an unpleasant odor from your exhaust pipe.

It is likely that you will not experience any symptoms in almost all cases except for the check engine light with P0420 code located within either the powertrain control module or engine control module (PCM).

What is the severity of P0420?

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

The only thing that could potentially happen is that the catalytic converter is so damaged that the catalytic converter’s internal parts come loose and block the exhaust flow, which is pretty unlikely to happen though.

However, the P0420 code trouble code may cause your car to emit more harmful emissions. It is important that you fix the problem as soon possible.

Which are some common P0420 diagnoses errors?

It is a common error to replace oxygen sensors without properly diagnosing the problem. The cause of this engine code is in most times the catalytic converter – which may get damaged by other issues with your car’s engine like misfires or a rich or lean air-fuel ratio.

You can check for other problems codes using your diagnostic scanner. If you find codes like P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308, you have a misfiring engine and the misfires may cause the P0420 error code. If this happens, first fix your misfires, then clear all codes to see if P0420 appears.

If you find DTC codes like P0171, P0172, P0174, or P0175, it means that your engine is running rich or lean. It could also be caused by a defective fuel injector or high or low fuel pressure. It is necessary to address the air-fuel mixture and check if it returns with the P0420 code.

If there is no problem with the engine in some instances, you may be able to try a catalytic converter cleaning before replacing your catalyst. We recommend that you choose one of our top-rated catalytic cleaners. There are so many additives available on the market.

Common causes of P0420 codes by car model

The P0420 code trouble code can be found in more car models than other. This trouble code is common in certain car models. Below is a listing of common car brand causes.

Keep in mind that these guidelines are not meant to be used as a guideline. You should always consult a doctor before you attempt to replace any component.

1. Chevrolet

This is usually caused by an exhaust leak in the catalyst. You may also experience a malfunctioning downstream oxygen sensor or catalytic converter.

Chevy engine’s are known to leak around the exhaust manifold so you should definitely take a look there if you can find some strange blowing sounds when your engine is running.

2. Toyota & Nissan

Bad catalytic converters are the main cause of P0420 problems in Toyota and Nissan automobiles. You can also make sure your engine doesn't burn oil.

The first thing to do is check for any exhaust or vacuum leaks. Check to make sure there is no blue smoke coming out of the exhaust pipe. If so, it’s a sign that you might have an issue with the crankcase ventilation or even something as bad as a damaged turbocharger or bad piston rings.

If there is no blue smoke, your catalytic convert is probably worn. Nissan cars may have similar problems.

2. Ford

Many Ford engines have a problem with the EGR, or vacuum leaks.

To check for trouble codes regarding the air-fuel combination, use a scanner that can scan the code memory. You can check for exhaust leaks, if all is well.

Diagnose and replace the catalytic converter if you can’t find any trouble codes or other problems with the air-fuel mixture.

3. Subaru & Honda

If you find a P0420 code in your Subaru or Honda, it’s most likely the catalytic converter to blame. An exhaust leak, or bad oxygen sensor can cause this code.

Vacuum leaks and other codes that are related to fuel mixtures should be checked. You should check for exhaust leaks prior to the catalytic conversion. The catalytic converter is the main problem in Subaru engines. This is due to their tendency to fail quickly on boxer engines.

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

This trouble code P0420 is caused by known issues with VAG vehicles. Check the check valves’ function under the intake and ensure the crankcase ventilation is free from dirt, causing the engine to burn oil, which clogs the catalytic converter.

Examine the exhaust for leaks near any flex pipe on the exhaust pipe. Also check for oxygen sensor codes. If there were no problems, you can replace the catalytic convert. It’s a widespread problem on both the 1.8T and the V6 petrol engines.

How can you identify the code P0420?

As mentioned above, P0420 can usually be caused by a defective catalytic convertor. Before replacing parts, you should first diagnose the problem using these methods.

1. Use an OBD2 scanner to look for trouble codes. First, fix any trouble codes that are related to fuel or ignition.
2. To see and interpret the O2 sensor voltage signals, check the live data. 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. You may have a problem with your catalytic converter if you receive similar readings.
3. The engine should be heated up and checked for temperature with the laser thermometer. If the engine is hot and there is no difference in temperature before and after the catalytic converter – your catalytic converter may not work as intended.
4. It is worth taking the pipe off the end that connects to the catalyst and inspecting the inside for any visual damage.
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. You should make sure that it's a genuine OEM Catalytic Converter and is installed correctly. If everything seems fine – replace the catalytic converter.

For more complex troubleshooting, you can use video:

What are the possible dangers to your catalytic converter

The catalytic converter reduces the three main causes of pollution produced through gasoline combustion – Nitrogen oxide(NOX), carbon monoxide, and hydrocarbons. These harmful substances are converted into more benign ones which are better for the health of our air and help to improve clean air quality.

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

• Mistakes
• Oil consumption
• Lunge leakage
• Leakage in intake
• Rich mixture
• Lean mixture
• PCM/ECM - Bad

What is the cost of DTC P0420 repairs? 

Due to the variety of causes for the code P0420, there's no set cost. It is usually caused by a defective catalytic converter that can cost between 500$ and 1500$.

Here is an estimate of the cost to repair the code. Prices include labor at a workshop and part costs. These prices do not cover diagnostic costs.

What parts can I take off to remove the code P0420?

It is not possible to just take out the trouble code. This will cause you other problems or trouble codes. But, you could reprogram the engine control unit to remove the catalytic converter’s monitoring system. Still, it’s not recommended because it will be illegal in most countries and states.

The catalytic convert can be removed completely if the user wants to reprogram its function. You should remember that you can't pass emission tests if the catalytic convert is removed. This could lead to legal problems.

You can also trick the engine control system by placing the rear oxygen sensor within a small tube. 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.

Commonly Asked Questions

Similar Codes

P0430 – Catalyst System Efficiency Below Threshold (Bank 2)