How to Fix Code P0420 for a Honda Accord? Causes

One of the most common trouble codes that car owners might encounter is the P0420 code. Suppose you drive a Honda Accord and have the “Check Engine Light” illuminated on your dashboard with a diagnostic code of P0420. In that case, it indicates a problem with the efficiency of the catalytic converter. This article will provide a step-by-step guide regarding how to fix code p0420 for a Honda Accord, including causes and symptoms.

code p0420 honda accord

What is P0420 Code Mean?

P0420 is the OBD-II generic code that suggests the catalytic converter in the engine bank 1 is not working efficiently. The catalytic converter’s primary role is to reduce harmful emissions by converting them into less harmful gases. If the converter isn’t working correctly, harmful pollutants might get released into the atmosphere.

What Could Cause a P0420 Code?

The code p0420 Honda Accord denotes “Catalytic Converter Efficiency Below Threshold (Bank 1),” signaling that the vehicle’s computer has detected reduced efficiency in the catalytic converter. Various factors can trigger this code, including:

1. Failed Catalytic Converter: The primary cause. Over time, catalytic converters can degrade due to internal wear or external damage.

2. Faulty Oxygen Sensors: Specifically, the downstream (post-catalytic converter) sensor. A malfunctioning oxygen sensor might give incorrect readings, indicating poor catalytic converter efficiency when the converter is okay.

3. Exhaust Leaks: Leaks, especially before the catalytic converter, introduce additional oxygen that can skew the readings of the oxygen sensors.

4. Rich or Lean Air-Fuel Mixture: An improper balance in the fuel mixture can lead to contaminants that the catalytic converter can’t efficiently process.

5. Engine Misfire: Misfires can send unburned fuel into the exhaust system, damaging the catalytic converter and altering its efficiency.

6. Engine Coolant or Oil Contamination: If coolant or oil leaks into the exhaust, it can contaminate the catalytic converter, hindering its efficiency.

7. Faulty Air/Fuel Ratio Sensor: This sensor, distinct from the oxygen sensors, monitors the mixture of air and fuel. Its malfunction can impact the catalytic converter’s efficiency.

8. Damaged or Failing Spark Plugs: These can result in unburned fuel entering the catalytic converter.

9. Worn-out or Damaged Catalytic Converter O2 Sensor Gasket: A damaged gasket can allow air to leak into the system, affecting readings and performance.

It’s essential to diagnose the cause behind a P0420 code thoroughly.

How to Fix Code P0420 for a Honda Accord?

Step-by-Step Troubleshooting:

Follow the below steps to fix the issue:

1. Visual Inspection:

Always begin with a thorough visual inspection. Check the exhaust system for damage or leaks. Any visible damage can be the root cause of the problem.

2. Check for Exhaust Leaks:

Leaks before the catalytic converter can introduce excess oxygen into the exhaust, which might skew the readings of the oxygen sensors. Repair any leaks you find.

3. Examine the Oxygen Sensors:

The DTC CodeP0420 often relates to the oxygen sensors, which measure the efficiency of the catalytic converter. Use an OBD-II scanner to check the readings. If the sensors provide inconsistent readings or appear faulty, consider replacing them.

4. Inspect the Catalytic Converter:

inspect catalytic converter

Look for any physical damage or discoloration. Overheating or contamination can reduce its efficiency. It might be clogged, damaged, or even contaminated by oil or coolant leaks.

5. Fuel System Check:

An overly rich or lean fuel mixture can affect catalytic converter efficiency. Check for any issues with the fuel injection system, fuel pressure, or even the condition of the spark plugs.

6. Update the ECU (Engine Control Unit) Software:

In some cases, the manufacturer might release updates to the vehicle’s software that can address the efficiency of the converter or how the system monitors it.

7. Reset the Code and Test Drive:

After performing any fixes, reset the code using the OBD-II scanner. Then, take your car on a test drive. The issue may have been resolved if the code doesn’t come back.

8. Replacement:

If the code returns after all these steps, it might be time to replace the catalytic converter. While this is an expensive repair, it’s essential for both the performance of your car and environmental considerations.

Fix P0420 Ford Code YouTube


  • Always use quality replacements designed for your specific Honda Accord model when replacing parts.
  • Regularly servicing your vehicle and ensuring optimal engine performance can help prolong the life of the catalytic converter.

Symptoms of P0420 code

The Honda DTC p0420 error code, which indicates a malfunction in the catalytic converter’s efficiency, usually manifests through several notable symptoms. Here’s a breakdown of the common signs you might notice when encountering this code:

1. Illuminated Check Engine Light: The most immediate sign is the illumination of the check engine light on your dashboard. This light generally signals various issues and, in this case, is triggered by the detection of an inefficient catalytic converter.

2. Reduced Fuel Efficiency: A compromised catalytic converter cannot process exhaust gases effectively, potentially leading to decreased fuel economy. You might find yourself visiting the fuel station more frequently.

3. Decreased Performance: Your vehicle might lose power, especially during acceleration. The car might feel sluggish, and the overall performance might be hampered.

4. Foul-Smelling Exhaust: A malfunctioning catalytic converter may not convert harmful gases into less harmful substances effectively, leading to a foul smell, akin to rotten eggs, coming from the exhaust.

5. Difficulty Starting: A damaged catalytic converter can cause starting issues in severe cases. The car might take longer to start, or you may experience rough starts.

6. Failed Emissions Test: One of the primary functions of a catalytic converter is to reduce vehicle emissions. A P0420 code can signal that your vehicle may fail an emissions test, as it might release higher levels of harmful gases into the atmosphere.

7. Heat and Overheating Issues: A failed catalytic converter can sometimes cause overheating issues, with the vehicle exhibiting higher than average temperatures, especially noticeable on the floorboard or center console.

8. Rattling Noises: You might notice a rattling noise coming from the undercarriage of the vehicle. This could be a sign that the internal honeycomb structure of the catalytic converter has broken down, leading to a noisy exhaust system.

If you notice any of these symptoms, conducting a thorough diagnostic check is recommended to pinpoint the exact cause and promptly remedy the issue.

Which O2 sensor causes P0420?

The DTC Code P0420 Honda Accord is primarily linked to the performance of the downstream oxygen sensor, also known as the post-catalytic converter sensor or O2 Sensor 2. This sensor monitors the oxygen content in the exhaust gases after passing through the catalytic converter. Its readings help in determining the efficiency of the converter.

A faulty or malfunctioning downstream O2 sensor can often trigger the P0420 code, indicating that the catalytic converter is not functioning optimally, when in fact, it could just be the sensor that needs replacement or cleaning. Always check this sensor first when troubleshooting a P0420 code.

Is P0420 Bank 1 or Bank 2?

The P0420 code pertains explicitly to “Catalytic Converter Efficiency Below Threshold (Bank 1).” In vehicles with a V-shaped or horizontally opposed engine, there are typically two banks: Bank 1 and Bank 2. Bank 1 is usually the side of the engine that contains the first cylinder or, in some configurations, the side of the engine with the odd-numbered cylinders.

Bank 2 typically refers to the opposite side. The P0420 code indicates an issue with the catalytic converter’s efficiency on Bank 1. If you encountered a similar problem with Bank 2, the OBD-II code would typically be code P0430. Always consult your vehicle’s manual or a trusted mechanic for specific details.

How Much Does It Cost to Fix a P0420 Code?

The cost to fix a P0420 Honda code can vary significantly based on the root cause of the problem and the make, model, and age of your vehicle. Here are some general estimates:

1. Oxygen Sensor Replacement: If the issue is due to a faulty oxygen sensor, the replacement cost can range from $50 to $150 for parts, plus $40 to $100 for labor. Depending on the vehicle, some sensors can be more expensive.

2. Catalytic Converter Replacement: This is often the priciest fix. The cost for a new catalytic converter can range from $300 to $2,000 (or even more for some luxury or specialized vehicles) for the parts. Depending on the vehicle’s complexity and local labor rates, labor can add an additional $100 to $300.

3. Exhaust Leak Repair: If there’s an exhaust leak, the repair cost could range from $100 to $300, depending on the leak’s severity and location.

4. Fuel Injector Service or Replacement: Addressing issues with the fuel system might cost between $50 for a simple cleaning and $500 or more for comprehensive injector replacement.

5. Software Update: In some cases, an ECU software update might be available, which might be covered under warranty, or it could cost up to a couple of hundred dollars.

6. Miscellaneous Fixes: Other minor fixes, such as replacing spark plugs, gaskets, or addressing minor leaks, can range from $50 to $500.

The total cost to address a P0420 code can range from as low as $100 for a minor fix to well over $2,500 for a high-end catalytic converter replacement. Always get multiple quotes and consult a trusted mechanic for an accurate diagnosis and cost estimate specific to your vehicle.


While a P0420 code might initially be daunting, understanding the potential causes and systematically troubleshooting can help pinpoint the issue. Whether it’s a minor fix like a sensor replacement or a more extensive repair like a catalytic converter replacement, addressing the issue ensures your Honda Accord remains in peak condition and environmentally friendly. Always consult a reliable mechanic if you are unsure about any aspect of the repair process.


Mr. Shafiqule Islam is a graduated Mechanical Engineer and has more than 15 years experience of repairing and maintenance of different brand vehicles like Toyota, Mitsubishi, Ford, Mercedes, BMW etc. He is also giving training to Mechanics. He has started writing to share his practical knowledge to Vehicle Owners, Drivers and Mechanics to keep their cars at best fit.

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