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P0420 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to clear p0420 code, and what other codes may show related to it.

One of the most frequent diagnostic trouble codes (DTCs) that car owners see is P0420. It means that Bank 1’s catalytic converter performance on Bank 1 has been identified by the engine control module (ECM). The catalyst may be at fault for this code, or an O2 sensor may be to blame for a false alert.

Understanding the significance of the P0420 code and how to resolve it are crucial. The catalytic converter is not functioning effectively, which can result in greater emissions and poor fuel economy, according to the code. Code P0420 may be saved for a number of reasons, such as a damaged catalytic converter or a rich or lean operating state. As a result, identifying the issue and correcting it might be difficult and need a deep knowledge of the car’s systems.

The P0420 code will be thoroughly explained in this article, along with its significance, causes, and solutions. Additionally, it will go through the signs of a broken catalytic converter and how to identify the issue. Readers will have a thorough grasp of the P0420 code and how to resolve it by the conclusion of this essay.


Catalyst System Efficiency Below Threshold (Bank 1)


The catalytic converter in your vehicle breaks down harmful pollutants created during the combustion cycles in your engine. Fine gold and platinum meshes filter exhaust fumes and reduce harmful emissions. To monitor proper airflow, the catalytic converter has two oxygen sensors: one upstream and one downstream.

A properly functioning upstream sensor should display readings that fluctuate from when the car is at operating temperature and running in a closed loop. Conversely, the downstream sensor should display static readings that hold steady except when there is a problem with the catalytic converter. Oxygen sensors with closely similar readings indicate the catalytic converter is not functioning as intended.

The tipoff  that something is amiss occurs when the voltage of the downstream oxygen sensor decreases. Usually it also begins to fluctuate like the upstream oxygen sensor, indicating that oxygen levels are too high and the Powertrain Control Module (PCM) stores a P0420 trouble code.


The P0420 error code may arise for a number of reasons. Among the most frequent causes are:

  • Inefficient catalytic converter: The efficiency of the catalytic converter may be reduced due to damage or wear and tear. This can be the result of ordinary wear and tear or exposure to extremely high or low temperatures.
  • Oxygen sensor malfunction: The oxygen sensor detects the oxygen content of the exhaust system and communicates this data to the ECM. The sensor may malfunction and transmit the ECM the wrong data, resulting in the P0420 error code.
  • Exhaust leaks: Air can enter the system through leaks in the exhaust system, which can cause the oxygen sensor to give false readings.
  • Fuel system difficulties: When the engine runs rich, it produces more pollutants. Fuel system faults might include malfunctioning fuel injectors or clogged fuel filters.
  • Problems with the ignition system can result in the engine misfiring, which raises emissions. Examples of ignition system problems include a bad spark plug or a broken ignition coil.

The P0420 error code is a warning that the catalytic converter is not operating properly, to sum up. Numerous things, such as a damaged catalytic converter, a malfunctioning oxygen sensor, exhaust leaks, problems with the fuel system, and ignition system problems, might cause it. To stop future harm to the vehicle, the fundamental problem must be quickly identified and fixed.


Here’s what to expect when driving a vehicle with a P0420 trouble  code:

  • Nothing at all. Generally a faulty catalytic converter doesn’t cause drivability issues (most common problem)
  • Check Engine Light is on due to P0420 trouble code
  • Lack of power when accelerating after the vehicle has warmed up
  • Difficulty getting the vehicle to reach speeds of 30-40 MPH
  • Sulfur or “rotten egg” smell coming from the exhaust pipe.


Tools Needed

A few tools are needed in order to troubleshoot the P0420 error code. These instruments include a vacuum gauge, a multimeter, and a scan tool. The diagnostic trouble codes (DTCs) and freeze frame information are retrieved using the scan tool. The oxygen sensors are tested with a multimeter, and the backpressure of the exhaust system is tested using a vacuum gauge.

Steps to Diagnose

The P0420 code can be diagnosed using the steps listed below:

  1. Use a scan tool to obtain the DTCs and freeze frame information. Information about the circumstances that led to the code being set may be learned from this data.
  2. Check the exhaust system for rust, damage, or leaks. The catalytic converter may malfunction due to any problems with the exhaust system.
  3. Utilize a multimeter to test the oxygen sensors. While the engine is running and the exhaust system is at working temperature, the sensors should be examined.
  4. Using a vacuum gauge, check the back pressure in the exhaust system. The engine should be operating at idle, and the gauge should be attached to the intake manifold.
  5. The catalytic converter could be defective and need to be changed even if the oxygen sensors and exhaust system backpressure are within specifications.

The P0420 error code can be brought on by a number of different problems, thus a comprehensive diagnostic is required to identify the underlying reason.

Common mistakes

What we observe the most often is technicians or DIYers who swap out the O2 sensor or catalytic converter without completing a full troubleshooting inspection on the vehicle. If another issue like a leaking fuel injector or oil contaminating the catalytic converter is the cause of the P0420 code, replacing an oxygen sensor or the entire catalytic converter needlessly is an expensive mistake.

How serious is this?

Few drivers will observe a noticeable difference if their vehicle has stored code P0420. The check engine light will be on, but not much else is obvious about the problem at first. That being said, failing to check out the trouble code and determine the root cause of the issue can seriously damage the catalytic converter and other components or systems if ignored for extended time periods.

The catalytic converter is an expensive repair regardless of the type of vehicle, so it is critically important to check trouble codes when the Check Engine Light comes on. Waiting to diagnose the problem or ignoring it can turn a minor fix into an expensive repair.

What repairs can fix the code?

The engine control module (ECM) has identified underperformance of the catalytic converter’s efficiency on Bank 1 when the P0420 code displays. A defective catalyst or a false warning brought on by an O2 sensor might both be to blame for this. There are two basic ways to solve this problem: do-it-yourself fixes and expert help.

DIY Solutions

You can attempt various do-it-yourself P0420 code fixes if you have some knowledge of auto repairs. Here are a few possibilities:

  • Replace the O2 sensor: Replacing the O2 sensor can resolve issues with P0420 codes that are brought on by malfunctioning sensors. To save time and money, be sure to swap out the appropriate sensor, which is the one located behind the catalytic converter.
  • Clean the catalytic converter: Sometimes a filthy or blocked catalytic converter might set up the P0420 error code. You might try cleaning it using a catalytic converter cleaner in this situation. However, keep in mind that this is not a permanent solution and may not be effective in all situations.
  • Check for exhaust leaks: The P0420 code can also be brought on by an exhaust leak. Look for leaks in the exhaust system, and if you find any, repair them.

Professional Assistance

It is advised to seek expert help if the DIY remedies don’t work or you don’t feel confident using them. Here are a few possibilities:

  • Visit a mechanic: A mechanic can identify the P0420 code’s primary cause and offer a fix. They could advise changing the catalytic converter, which is a pricey but long-lasting remedy.
  • Use a specialized service: A few businesses provide specialist services to resolve the P0420 error. The efficiency of the catalytic converter is cleaned and restored using cutting-edge technology. These services, meanwhile, can be pricey and not always effective.

In conclusion, resolving the P0420 error code necessitates locating the underlying cause of the issue and performing the necessary steps. Whether you choose for a do-it-yourself approach or seek out expert help, be sure to select reputable sources and adhere to the suggested procedures.

Preventing P0420 Code

The P0420 code may be avoided by performing routine maintenance and using safe driving practices. Observe the following advice:

Regular Maintenance

To maintain your car in good working order and avoid the P0420 code, routine maintenance is crucial. Following are some suggestions for maintenance:

  • Regularly inspect and replace the air filter. The engine may run rich due to a filthy air filter, which might harm the catalytic converter.
  • Spark plugs should be frequently checked and replaced. Spark plugs that are worn out or fouled can result in incomplete combustion, which can harm the catalytic converter.
  • Regularly inspect and replace the oxygen sensors. The engine may run rich due to faulty oxygen sensors, which may harm the catalytic converter.
  • Utilize the correct fuel grade. Using fuel of a poorer quality can make the engine run rich, which can harm the catalytic converter.

Driving Habits

Driving practices can also affect whether the P0420 code appears. Here are some guidelines for safe driving:

  • Avoid abrupt starts and pauses. The engine may run rich as a result of abrupt pauses and starts, which may harm the catalytic converter.
  • Avoid maintaining a high pace when driving. Driving at a high rate of speed can make the engine run hot, which can harm the catalytic converter.
  • Avoid spending a lot of time in stop-and-go traffic. Stop-and-go traffic may result in an overheated engine, which may harm the catalytic converter.
  • Avoid packing the car too full. A car that is overloaded may need more effort from the engine, which might harm the catalytic converter.

You can assist avoid the P0420 code from occurring and keep your car running smoothly by paying attention to these suggestions.


The P0420 error code denotes that the engine control module has detected an issue with the catalytic converter’s effectiveness. A damaged, inefficient catalytic converter or a false alert set off by a sensor like the downstream or upstream oxygen sensor are also potential causes of code P0420.

To prevent more harm to the emission system and potential engine damage, if the catalytic converter is the issue, it is critical to treat the situation as quickly as possible. A malfunctioning catalytic converter can increase pollution since it is an essential part of the exhaust pipeline’s system for lowering gas emissions.

A P0420 code requires significant troubleshooting, and there is a vast list of potential reasons. To prevent needless repairs, it is essential to determine the problem’s underlying source. To accurately diagnose and resolve the problem, it is advised to seek the assistance of a skilled technician.

P0420 Code – What Does It Mean & How To Fix It