Question

How to diagnose and fix a P0420 code on a Subaru? 

The P0420, one of the most common yet overlooked codes on a Subaru is a challenge to diagnose and it requires professional help. The initial diagnosis, however, can be done by yourself using a code scanner. To diagnose it further, you should: 

• Inspect the catalytic converter for any damage or contamination. Perform a pressure test to determine whether it’s functioning properly 
• Check whether the oxygen sensors are giving accurate readings. A malfunctioning 02 sensor can also cause a P0420 on a Subaru 
• Check the exhaust system for any leaks, damage or corrosion 
• The fuel system should be thoroughly investigated. Ensure that the fuel injectors are working properly and the fuel filter is unclogged.  
• Inspect the EGR system 
• While you are at it, get your catalytic converter cleaned 

Diagnosing a P0420 on a Subaru accurately is half the battle won and fixing it is comparatively easier. Here’s what you’ll need to do: 

• Locate and repair any exhaust leaks 
• Replace any faulty sensors 
• Replace spark plugs, spark plug wires, and the air filter to ensure the engine is running efficiently. An efficiently running engine means a functioning catalytic converter. 
• If the problem persists, it’s probably time to replace your catalytic converter. Make sure to use original equipment manufacturer parts and not the aftermarket ones.