What is the code P0420 on a Subaru Tribeca?

The Subaru Tribeca's P0420 code is a diagnostic trouble code that indicates an issue with the vehicle's catalytic converter. This code is triggered when the engine control module (ECM) detects that the catalytic converter is not efficiently converting the engine's exhaust emissions into less harmful substances.

What is the P0420 Code?

The P0420 code is a generic diagnostic trouble code that applies to many vehicles, not just the Subaru Tribeca. It is triggered when the ECM detects that the catalytic converter's efficiency is below a certain threshold. The catalytic converter is responsible for converting harmful exhaust emissions, such as carbon monoxide and unburnt hydrocarbons, into less toxic substances before they are released into the atmosphere.

Causes of the P0420 Code in Subaru Tribecas

There are several potential causes for the P0420 code in a Subaru Tribeca, including:

• Faulty catalytic converter: The catalytic converter may be damaged, clogged, or simply worn out over time, reducing its efficiency.


• Exhaust leaks: Leaks in the exhaust system can allow unburnt fuel and air to bypass the catalytic converter, reducing its effectiveness.


• Oxygen sensor issues: The oxygen sensors that monitor the catalytic converter's performance may be malfunctioning, causing the ECM to incorrectly detect a problem.


• Engine problems: Issues with the engine, such as misfiring or poor fuel combustion, can also lead to the catalytic converter becoming less efficient.

If you receive a P0420 code in your Subaru Tribeca, it's important to have the vehicle diagnosed by a qualified mechanic to determine the root cause and address the issue.

Addressing the P0420 Code

The most common solution for a P0420 code in a Subaru Tribeca is to replace the catalytic converter. However, it's important to first diagnose the underlying cause to ensure the new catalytic converter doesn't also become inefficient. Other potential solutions may include repairing exhaust leaks, replacing faulty oxygen sensors, or addressing any engine-related problems.

If the P0420 code persists after addressing the initial issue, it may be necessary to replace the entire exhaust system, including the catalytic converter, to ensure proper emissions control and engine performance.

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

Does P0420 mean I need a new catalytic converter?

A P0420 code doesn't always mean you need a new catalytic converter, though it is a common cause. The code indicates that the catalytic converter's efficiency is below the required threshold, which can also be caused by issues like exhaust leaks, a faulty oxygen sensor, or problems with the engine's fuel system. Proper diagnosis is crucial to determine the root cause before replacing the converter. 
This video explains the P0420 code and its relation to the catalytic converter: 1mEasyAutoFixYouTube · Sep 28, 2022
Other causes of a P0420 code

• Exhaust leaks: A leak in the exhaust system before the catalytic converter can allow extra oxygen in, which the oxygen sensors will read as the converter not working properly. 
• Faulty oxygen sensors: The P0420 code is triggered when the front and rear oxygen sensors provide similar readings, indicating the converter is not performing its function. If one of the sensors is faulty, it can lead to this code. 
• Engine performance problems: Issues like engine misfires, contaminated fuel, or a bad fuel mixture can damage the catalytic converter over time and trigger the code. 

How to diagnose the issue

• Check for other codes: Use an OBD-II scanner to check for any other related trouble codes that could point to a different problem. 
• Inspect for exhaust leaks: Look for any leaks in the exhaust system, especially around the catalytic converter itself. 
• Analyze oxygen sensor data: A mechanic can use a scanner to monitor the live data from the oxygen sensors. The readings will show if the converter is functioning efficiently. 
• Check engine health: Ensure the engine is running properly with no misfires or issues with the fuel system. 

You can watch this video to learn how to check for exhaust leaks: 56sTRQYouTube · Aug 18, 2023

What causes the P0420 code on a Subaru?

The Subaru P0420 code typically points to low catalytic converter efficiency, but the cause is often a failing or contaminated O2 sensor (oxygen sensor) or, less commonly, an exhaust leak or a failing catalytic converter itself. Other potential issues include an oil leak in the spark plug wells due to failing valve cover gaskets and problems with the engine's PCV or EGR valves. 
Common causes

• Failing O2 sensor: This is one of the most frequent culprits, especially on older Subaru models. The downstream O2 sensor (after the catalytic converter) may provide incorrect data, or the upstream (air/fuel ratio) sensor could be faulty. 
• Failing catalytic converter: The converter itself can become clogged or degraded over time, particularly after high mileage or if poor-quality aftermarket parts were used previously. 
• Exhaust leaks: Leaks in the exhaust system, even a small crack, can allow air in, which can lead to an incorrect reading by the O2 sensors and trigger the code. 
• Oil leaks: Oil entering the spark plug wells from a failing valve cover gasket can foul plugs and lead to misfires, which in turn can cause the P0420 code. 
• Engine issues: Problems with the engine's PCV valve, EGR valve, or general fuel system performance can also affect emissions and trigger the code. 

Subaru-specific checks

• Spark plug wells: Check the spark plug wells for oil leaks, which are common due to old valve cover gaskets. 
• PCV valve: Inspect the PCV valve to ensure it's not rattling and is functioning correctly. 
• EGR valve: Verify that the EGR valve is moving freely. 
• Ground straps: Examine chassis ground straps, as Subaru vehicles have specific points that can corrode or fail. 
• Catalytic converter quality: If a replacement is needed, consider using an OEM part, as cheap aftermarket converters have a high rate of failure on Subarus due to the size and performance requirements of the converter. 

What is the code P0420 on a 2008 Subaru Tribeca?

2008 Subaru Tribeca P0420 and P0024 Code Diagnosis
P0420 indicates catalytic converter efficiency below threshold, often caused by failing cats or faulty O2 sensors. Start by testing upstream and downstream oxygen sensors for accuracy before replacing the catalytic converter.