What is the code P0420 on a 2008 Honda Ridgeline?

On a 2008 Honda Ridgeline, P0420 indicates the catalytic converter’s efficiency is below the expected threshold for Bank 1. This often triggers the check engine light and prompts inspection of the exhaust after-treatment system.

In practical terms, P0420 is an emissions-related code that signals the vehicle’s catalytic converter is not reducing pollutants as effectively as the factory specification requires. For the Ridgeline’s V6 setup, Bank 1 refers to one side of the exhaust pathway, where the downstream oxygen sensor monitors catalytic performance. A worn or failing catalytic converter, a faulty downstream O2 sensor, exhaust leaks, or wiring issues can all set this code. Addressing P0420 promptly helps maintain emissions compliance, fuel economy, and engine health over time.

What P0420 Means for the 2008 Ridgeline

The P0420 code is triggered when the engine control module detects insufficient catalytic converter efficiency based on comparison between upstream and downstream oxygen sensor signals. A healthy catalyst should reduce the amount of pollutants in the exhaust, causing the downstream sensor to read a different value than the upstream sensor. If the downstream sensor reads in a similar range to the upstream sensor, the PCM may set P0420.

Common causes of P0420 on a 2008 Ridgeline

The following items are frequently associated with P0420 on this model. These are starting points for inspection and diagnostic testing.

• Aging or failing catalytic converter


• Faulty downstream oxygen sensor (sensor 2) after the catalyst


• Exhaust leaks before or around the catalytic converter


• Wiring or heater circuit problems for oxygen sensors


• Engine running rich or lean due to fuel system issues or ignition problems


• Engine misfire or unburned fuel washing the catalyst

Note: These are common causes and not a definitive diagnosis. A proper diagnostic using live data and possibly a physical inspection is required to confirm the root cause.

How to diagnose P0420 on a 2008 Ridgeline

Use the following diagnostic steps to determine the root cause. The steps assume access to a quality OBD-II scanner and basic mechanical tools. Start with the simplest checks and progress to more involved tests as needed.

1. Verify the code and examine live data: Confirm P0420 is present and review Bank 1 Sensor 1 (upstream) and Bank 1 Sensor 2 (downstream) readings. Look for a consistent pattern of readings and check for any other related codes (misfires, fuel trim, etc.).


2. Inspect for exhaust leaks: Check around the catalytic converter, gaskets, clamps, and piping for leaks that could skew sensor readings.


3. Test sensors and wiring: Inspect O2 sensor connectors and wiring for damage or corrosion. Test the downstream O2 sensor’s response and heater circuit if applicable; consider replacing a faulty sensor if readings are abnormal and other causes are ruled out.


4. Evaluate the catalytic converter: If sensor data indicates the converter isn’t effectively reducing emissions, you may need to assess catheter health via temperature readings (pre- versus post-cat) or a backpressure test. A high-mileage Ridgeline often has a worn converter contributing to P0420.


5. Check engine operation: Ensure there are no misfires, vacuum leaks, or fuel delivery issues that could overload the catalyst and trigger P0420.


6. Implement repairs and re-test: After performing lower-risk repairs (sensor replacement, leak fixes), clear codes and drive normally while monitoring with the scan tool. If P0420 recurs, consider replacing the catalytic converter or addressing any persistent sensor faults.

Professional diagnostics may include more advanced tests and emissions equipment checks, especially if the vehicle is subject to emissions inspections or if DIY procedures do not resolve the code.

Costs and repair options

Repair costs for P0420 can vary widely depending on the root cause and whether OEM or aftermarket parts are used. The following rough ranges can help you budget, but exact prices depend on location, labor rates, and parts choices.

• Downstream O2 sensor replacement: typically $100–$300 for parts, plus labor


• Catalytic converter replacement (Bank 1): commonly $900–$2,500 depending on OEM vs aftermarket and whether the cat is integrated into the exhaust; labor adds to the total


• Exhaust leak repairs (pipes, gaskets, clamps): commonly $50–$300 depending on the leak's location and severity


• Sensor wiring repairs: cost varies with labor and parts, generally under a few hundred dollars

Costs can vary significantly by shop and region. It’s wise to obtain multiple estimates and consider sequential repairs (starting with sensors or leaks) before replacing the catalytic converter, as some P0420 cases resolve after addressing simpler issues.

Summary

P0420 on a 2008 Honda Ridgeline signals that the catalytic converter’s efficiency is below the required threshold for Bank 1. The issue is commonly caused by a worn catalyst or a faulty downstream oxygen sensor, though exhaust leaks, wiring problems, or engine/fuel-system conditions may also trigger it. A methodical diagnostic approach—start with data checks, inspect the exhaust and sensors, and then address the simplest fixes before considering a catalytic converter replacement—helps minimize unnecessary repairs. If resolved, you should see the check engine light turn off and experience improved emissions performance and fuel economy.

What is the most common cause for a P0420 code Honda?

This trouble code is triggered when the on-board computer detects that the bank 1 of the catalytic converter is underperforming. A faulty catalytic converter (usually due to overheating and contamination) is the most common reason why this code is logged.

How do you fix P0420 catalyst system efficiency below threshold bank 1?

To fix the P0420 code, first diagnose the cause, as it could be a failing catalytic converter, a faulty oxygen sensor, an exhaust leak, or an engine issue like a misfire or bad injector. Fix any other related DTCs or issues first, then address the most likely culprits: replace the O2 sensor or the catalytic converter and repair any exhaust leaks. 
This video demonstrates how to diagnose the P0420 code: 59sTRQYouTube · Aug 18, 2023
Diagnostic steps

1. Use an OBD-II scanner: Connect a scanner to view live data. Use the graph function to check oxygen sensor output. A healthy downstream O2 sensor should have a steady, low reading, while a bad sensor will fluctuate similarly to the upstream sensor. 
2. Check for exhaust leaks: Perform a visual inspection for any signs of leaks in the exhaust system, especially near the oxygen sensors and catalytic converter. A simple test involves loosening the bolts in front of the converter to see if engine power improves, which can indicate a clogged converter, say these YouTube videos https://www.youtube.com/watch?v=45J72GHAtp8 and. 
3. Inspect the catalytic converter: Tap the converter with a rubber mallet. If you hear rattling, the internal structure is likely broken. 
4. Look for other engine problems: Check for other codes and address them first, such as misfires, which can damage the converter. A clogged fuel injector or a dirty air filter can also trigger this code. 

This video explains the relationship between the catalytic converter and fuel trim: 57sRisloneYouTube · Mar 22, 2022
Potential fixes

• Replace the O2 sensor: The upstream or downstream oxygen sensors are a common cause of P0420. Replacing a faulty O2 sensor can sometimes fix the code. 
• Repair exhaust leaks: Fix any leaks found during the inspection. 
• Replace the catalytic converter: If the converter is clogged, damaged, or the other steps don't work, replacing it is often the final solution. 
• Address engine issues: If there are misfires or leaky injectors, repair them. A clogged fuel injector can contaminate the converter. 
• Use a catalytic converter cleaner: If the converter is simply clogged, a cleaning additive might resolve the issue, especially if you use it with the vehicle on the highway. 

You can watch this video to learn how to replace the catalytic converter: 46sCtrlAltDefeatTVYouTube · Apr 4, 2018

What is the most common fix for the code P0420?

The most common fixes for a P0420 code involve addressing issues with the catalytic converter and oxygen sensors, with replacing the catalytic converter being the most common, but often expensive, solution. However, the underlying cause is often less severe, and the fix could be as simple as replacing the downstream oxygen sensor, repairing an exhaust leak, or addressing a misfire from faulty spark plugs or injectors. Thoroughly diagnosing the root cause before replacing parts is crucial to avoid unnecessary costs.
 
This video explains the meaning of P0420 code and how to fix it with catalytic converter cleaner: 59snondaYouTube · Oct 31, 2021
Potential fixes for P0420

• Diagnose and fix the root cause: Before replacing the catalytic converter, identify the true problem. If there are engine issues like misfires or fuel system problems, fix those first. These can damage the new converter and cause the code to return. 
• Replace the oxygen sensors: A faulty or aged oxygen sensor can give false readings. The downstream sensor is a common culprit, and replacing it can sometimes fix the problem at a lower cost. 
• Repair exhaust leaks: Leaks in the exhaust system, even small ones, can cause inaccurate oxygen sensor readings. Check the exhaust manifold and pipes for cracks and repair them. 
• Replace the catalytic converter: If diagnostics confirm the converter is failing, it will need to be replaced. It is best to use an OEM replacement to ensure it meets the vehicle's specifications. 
• Clean the catalytic converter: In some cases, a fuel or intake system cleaning additive might remove carbon buildup from the converter, restoring its efficiency. 

This video demonstrates how to use a catalytic converter cleaner to address the P0420 code: 1mCrushr ReviewsYouTube · May 31, 2024
Before you begin

• Get a proper diagnosis: Use a professional scan tool or a mechanic to check for other codes and analyze live data from the oxygen sensors to pinpoint the exact issue. 
• Check for underlying engine problems: Ensure the engine is in good health. Worn-out spark plugs, faulty fuel injectors, or vacuum leaks can all lead to a P0420 code and can damage a new catalytic converter. 

What is the code P0420 on a Honda Ridgeline?

A Honda Ridgeline P0420 code indicates the catalytic converter on Bank 1 is not functioning efficiently, likely due to a failing converter or faulty oxygen sensors. To fix this, first check for exhaust leaks and inspect or replace the oxygen sensors, as they are cheaper and a faulty sensor can cause a false code. If sensors and exhaust are fine, the catalytic converter itself is the probable cause and will need to be replaced. 
This video explains the causes and solutions for P0420 code: 58sEasyAutoFixYouTube · Sep 28, 2022
Diagnosis and troubleshooting

• Check for exhaust leaks: Inspect the exhaust system for any leaks, which can trigger the code. 
• Inspect oxygen sensors: The upstream and downstream oxygen sensors are common culprits. A faulty sensor can send incorrect data to the engine control module (ECM). 
  • Use a scan tool to check the voltage readings of the downstream O2 sensor. A healthy converter will show a relatively stable, low-voltage signal, while a bad one will show rapid fluctuations. 
• Check for other issues: Rule out other potential problems, such as misfires, fuel trim issues, or a damaged exhaust manifold or muffler. 

Potential fixes

• Replace oxygen sensors: If a sensor is faulty, replacing it is a cheaper first step and may resolve the issue. 
• Repair exhaust leaks: Fix any leaks in the exhaust system. 
• Replace the catalytic converter: If the sensors are working correctly and there are no leaks, the catalytic converter is the most likely cause. Replacing it is a more expensive fix, but necessary to resolve the code.