#OBD Codes

P0420 – Catalyst system, bank 1. Efficiency below threshold

P0420 – Catalyst system, bank 1. Efficiency below threshold
Back to OBD codes library

Technical definition

Catalyst system, bank 1. Efficiency below threshold.

What does P0420 mean?  

The code P0420 means the catalytic converter on Bank 1 isn't working as efficiently as it should. This problem often comes from damage to the exhaust system or issues with the fuel system. These issues result in an insufficient amount of oxygen leaving the catalytic converter.

In engines with two cylinder heads (like V6, V8, V10, W8), "Bank 1" indicates the side with the first cylinder. 

The catalytic converter is a part of the vehicle's exhaust system. Its primary function is to convert harmful emissions into less harmful substances before they exit the vehicle. Specifically, it turns unburned fuel and carbon monoxide (CO) into carbon dioxide (CO2) and water (H2O). It also reduces nitrogen oxides (NOX) back into nitrogen. 
 ChatGPT The image shows a cutaway of a catalytic converter. Exhaust gases enter from the left, pass through a honeycomb structure, and exit as less harmful substances like water vapor, nitrogen, and carbon dioxide on the right.

Two oxygen (O2) sensors monitor this conversion process: one before the catalytic converter (upstream) and one after (downstream). If these sensors detect similar oxygen levels on both sides of the converter, it suggests that the converter isn't working efficiently. This inefficiency triggers the P0420 code and causes the check engine light to turn on.

How serious is this DTC?

P0420 diagnostic trouble code (DTC) is moderately severe. It can cause slightly decreased fuel efficiency and engine performance, but the vehicle can still operate. However, it's important to note that it may lead to the car failing emissions test.

Can I still drive with P0420?

Yes, you can still drive if your car shows a P0420 code, but you should visit a mechanic soon. Even though your car might seem okay except for the check engine light flashing, driving like this for too long could harm the catalytic converter and related systems.

Symptoms of this DTC

Symptoms associated with the P0420 code include:

Common causes of this DTC 

Common causes for the P0420 code include:
  • Bad or clogged catalytic converter 
  • Cracked exhaust manifold 
  • Exhaust leaks before the catalytic converter 
  • Malfunctioning oxygen sensor 
  • Worn out spark plugs 
  • Engine misfiring 
  • Wrong fuel type 
  • Fuel injectors leakage 
  • Faulty fuel pump 
  • Wiring damage 
  • Malfunctioning engine coolant temperature sensor 
  • Faulty spark plugs or ignition coils 
  • Clogged air or fuel filter 
  • Loose fuel tank cap 
 ChatGPT A close-up of a cracked exhaust manifold on a car engine, showing a visible fracture and surface rust.

How to troubleshoot this DTC

With the right diagnostic tools, such as OBDeleven, and some knowledge, identifying the cause of a P0420 code can be straightforward.

This code usually indicates a problem with catalyst efficiency. However, a common mistake is to replace parts before fully understanding the cause. For example, replacing the converter without fixing the underlying problem can cause repeated damage to the new converter.  
To avoid wasting money repairing components that aren’t faulty, use these tips: 
Scan fault codes 
Use an OBD code reader to quickly pinpoint engine problems. Don't just focus on the P0420 code. You should also look for other codes in the engine's control unit. These may indicate related problems such as rich or lean fuel mixture, misfiring, damaged wiring, or faulty injectors. 
Clear the code and test drive
Use your scanner to clear fault codes. Then, drive your car for at least 15 minutes to fully warm up the engine – some issues only show up when the engine is hot.

After the drive, scan the engine again to check if the same problems reappear. If the issues persist, this indicates there's a real problem that needs further attention. This step helps determine whether the issue is ongoing or was just a one-time occurrence.

Check rear oxygen sensor voltage

Check the live data for the rear oxygen sensor voltage. Typical readings should range between 0.4 to 0.5 volts under normal conditions. Fluctuating or abnormal readings can indicate an issue with the catalytic converter.

Check other live data parameters

To identify potential causes of the P0420 issue, it’s helpful to examine other related live data parameters. Consider looking at:
  • Long/Short fuel trim values. Check if these values are within the normal range of -5% to +5%. Deviations from this range can indicate whether the engine is getting too much or too little fuel, potentially affecting the catalytic converter’s performance. 
  • Exhaust temperature. The catalytic converter operates effectively within a specific range of temperature. Exceeding 870°C (1600°F) can damage the catalyst material, leading to melting or cracking of the internal ceramic substrate and making the converter ineffective. 
  • Engine misfire data. Misfires can cause unburned fuel to damage the catalytic converter. Addressing misfires promptly is crucial for preventing further issues. 
  • Mass Air Flow (MAF) sensor data. This sensor measures the amount of air entering the engine. Incorrect readings can disrupt the engine’s air-fuel balance, impacting the catalytic converter. 
  • Fuel injectors condition. Make sure the fuel injectors are working properly. Malfunctioning injectors can lead to incomplete combustion and damage to the catalytic converter. 
Checking these parameters can help you understand why you might be having issues and what you might need to fix to get your car running right again.

Physical examination of the exhaust system 
Take a close look at your car's exhaust system to check for any damage. Look for holes, rust, or leaks that could be causing problems. It's especially important to check the exhaust manifold carefully, as it's a common place for leaks to occur. Identifying these issues early can help prevent more serious damage and keep your car running smoothly.


What tools are necessary for a DIY diagnosis of the P0420 code?

To diagnose this issue, you will need an OBD scanner to read fault codes and live data parameters from your vehicle's engine control unit. A digital multimeter is also handy if you suspect sensor or wiring issues.

Using a car lift can make it easier to examine the exhaust system by improving access and visibility. It will help you spot any damage or leaks more easily. In addition, an infrared thermometer can be useful to check the temperatures of the catalytic converter and exhaust system. 
How can I prevent triggering a P0420 code? 
You can prevent the P0420 code by regularly replacing spark plugs, fuel, and air filters, using the correct type of fuel, and checking for exhaust leaks. Additionally, using an OBD scanner to detect problems early can help avoid serious issues down the road.
What environmental impact does a malfunctioning catalytic converter have? 
A malfunctioning catalytic converter increases harmful emissions, contributes to air pollution, and has a negative impact on public health 
What are the risks of ignoring the P0420 code for a long period? 
Ignoring code P0420 can lead to serious problems. These include catalytic converter damage, failed emissions tests, and costly replacement. It can also result in reduced performance and fuel efficiency. 
Can aftermarket mods trigger a P0420 code in my car? 
Yes, aftermarket modifications such as changes to the exhaust system, use of non-standard oxygen sensors, or changes to the engine's air-fuel balance can cause a P0420 code. 
Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.

Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.