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P0420 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to clear p0420 code, and what other codes may show related to it.


Catalyst System Efficiency Below Threshold (Bank 1)


The catalytic converter in your vehicle breaks down harmful pollutants created during the combustion cycles in your engine. Fine gold and platinum meshes filter exhaust fumes and reduce harmful emissions. To monitor proper airflow, the catalytic converter has two oxygen sensors: one upstream and one downstream.

A properly functioning upstream sensor should display readings that fluctuate from when the car is at operating temperature and running in a closed loop. Conversely, the downstream sensor should display static readings that hold steady except when there is a problem with the catalytic converter. Oxygen sensors with closely similar readings indicate the catalytic converter is not functioning as intended.

The tipoff  that something is amiss occurs when the voltage of the downstream oxygen sensor decreases. Usually it also begins to fluctuate like the upstream oxygen sensor, indicating that oxygen levels are too high and the Powertrain Control Module (PCM) stores a P0420 trouble code.


There is an extensive list of causes for a P0420 code, and the troubleshooting procedure is long as well. The following list provides a summary of potential equipment failures that can cause a P0420:

  • Damaged/leaking muffler
  • Damaged/leaking exhaust manifold
  • Damaged/leaking exhaust pipe
  • Engine misfire
  • Oil contamination in the catalytic converter
  • Faulty/broken catalytic converter (the most common reason for a P0420)
  • Faulty engine coolant sensor
  • Faulty front O2 sensor
  • Faulty rear O2 sensor
  • Damaged O2 sensor wiring
  • Damaged/disconnected O2 sensor connectors
  • Leaking fuel injector
  • High fuel pressure
  • Using the wrong type of fuel for the vehicle (leaded fuel will damage catalytic converters designed to run on unleaded fuel)


Here’s what to expect when driving a vehicle with a P0420 trouble  code:

  • Nothing at all. Generally a faulty catalytic converter doesn’t cause drivability issues (most common problem)
  • Check Engine Light is on due to P0420 trouble code
  • Lack of power when accelerating after the vehicle has warmed up
  • Difficulty getting the vehicle to reach speeds of 30-40 MPH
  • Sulfur or “rotten egg” smell coming from the exhaust pipe.

Also read: How to Clean Your Catalytic Converter Without Removing It?


Here’s how a qualified technician checks out a P0420 code to fix the problem the first time:

  • Pull all trouble codes using an OBD-II scanner, here’s a great one we use daily, clear codes and restart the vehicle
  • Check lives downstream (rear) oxygen sensor data. The downstream oxygen sensor voltage reading should be steady.If it’s not, there is a problem with either the downstream oxygen sensor or a problem with the catalytic converter.
  • Pull the downstream O2 sensor and check it for signs of excessive wear or damage
  • Check all O2 sensor wiring and connectors
  • Diagnose any other codes that could be causing the P0420 trouble code (there are a few related issues that could cause a P0420, especially the fuel regulation and oil pressure systems).
  • Fix any misfires, ignition problems, and/or fuel system problems as needed
  • Take a test drive and view the freeze-frame data to verify the voltage remains constant or fluctuates (remember, fluctuating voltage means there’s a problem with the sensor or converter).
  • Decision time: swap the O2 sensor or swap out the converter.
  • See if there are any available updates of the PCM if they determine the catalytic converter has failed. To ensure proper function, the PCM needs to have its software updated after the catalytic converter has been replaced.

Common mistakes

What we observe the most often is technicians or DIYers who swap out the O2 sensor or catalytic converter without completing a full troubleshooting inspection on the vehicle. If another issue like a leaking fuel injector or oil contaminating the catalytic converter is the cause of the P0420 code, replacing an oxygen sensor or the entire catalytic converter needlessly is an expensive mistake.

How serious is this?

Few drivers will observe a noticeable difference if their vehicle has stored code P0420. The check engine light will be on, but not much else is obvious about the problem at first. That being said, failing to check out the trouble code and determine the root cause of the issue can seriously damage the catalytic converter and other components or systems if ignored for extended time periods.

The catalytic converter is an expensive repair regardless of the type of vehicle, so it is critically important to check trouble codes when the Check Engine Light comes on. Waiting to diagnose the problem or ignoring it can turn a minor fix into an expensive repair.

What repairs can fix the code?

  • Replace/repair muffler/muffler leaks
  • Replace/repair exhaust manifold/exhaust manifold leaks
  • Replace/repair exhaust pipe/exhaust pipe leaks
  • Replace the catalytic converter
  • Replace engine coolant temperature sensor
  • Replace upstream or downstream O2 senso​​​​​rs.
  • Repair/replace damaged O2 sensor wiring
  • Repair/replace damaged O2 sensor connectors
  • Replace or repair fuel injector(s) that are leaking
  • Diagnose and repair engine misfires
  • Diagnose and repair other PCM trouble codes that pop up with P0420

Related codes

None listed.


The most important takeaway from this summary of P0420 is to pull codes for your vehicle as soon as you see your Check Engine Light come on. Take it to a shop or use a personal scanner, but record the results to share with your technician later. Failing to diagnose and repair the problem quickly and correctly can lead to expensive repairs. Don’t ignore trouble codes and hope for the best.

P0420 code – what does it mean & how to fix it