Aging or degraded catalytic converter that has lost its ability to store and release oxygen; A faulty downstream oxygen sensor that is slow to respond or has a contaminated tip; Exhaust leaks before or near the downstream oxygen sensor, allowing outside air to affect readings; Engine misfires or rich fuel mixtures that have overheated or contaminated the catalyst; Oil or coolant consumption burning through the engine and poisoning the catalyst; A failing upstream oxygen sensor that is providing inaccurate feedback to the PCM; Damage to the catalytic converter from road debris or physical impact

DTC codes / P04xx · Auxiliary emission controls / P0420

P0420Catalyst System Efficiency Below Threshold (Bank 1)

Q: What does code P0420 mean?

Catalyst System Efficiency Below Threshold (Bank 1). P0420 means the powertrain control module (PCM) has determined that the bank 1 catalytic converter is not cleaning the exhaust as well as it should, based on oxygen sensor readings before and after the converter. The PCM compares the switching activity of the upstream and downstream oxygen sensors and sets this code when the downstream sensor mirrors the upstream sensor too closely, indicating reduced catalyst efficiency.

P0420 means the powertrain control module (PCM) has determined that the bank 1 catalytic converter is not cleaning the exhaust as well as it should, based on oxygen sensor readings before and after the converter. The PCM compares the switching activity of the upstream and downstream oxygen sensors and sets this code when the downstream sensor mirrors the upstream sensor too closely, indicating reduced catalyst efficiency.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaAuxiliary emission controls

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
A slight decrease in fuel economy as the PCM may adjust fuel trim to compensate
Occasional rough idle or hesitation if the catalyst is severely degraded
A sulfur or rotten egg smell from the exhaust in some cases
Failed emissions inspection due to increased tailpipe emissions
In rare cases, a noticeable loss of power under acceleration

Common causes, most likely first

Aging or degraded catalytic converter that has lost its ability to store and release oxygen
A faulty downstream oxygen sensor that is slow to respond or has a contaminated tip
Exhaust leaks before or near the downstream oxygen sensor, allowing outside air to affect readings
Engine misfires or rich fuel mixtures that have overheated or contaminated the catalyst
Oil or coolant consumption burning through the engine and poisoning the catalyst
A failing upstream oxygen sensor that is providing inaccurate feedback to the PCM
Damage to the catalytic converter from road debris or physical impact

How to diagnose and fix it

Inspect the entire exhaust system for leaks, especially around the downstream oxygen sensor and converter flanges
Use a scan tool to check for other stored codes, particularly misfire or fuel trim codes, and address them first
Graph the upstream and downstream oxygen sensor signals to confirm the downstream sensor is switching similarly to the upstream sensor
Test the downstream oxygen sensor for proper response and replace it if it is sluggish or contaminated
Perform a backpressure test to check for a clogged catalytic converter
If the converter is confirmed inefficient, replace it with a quality unit that meets emissions requirements
After repairs, clear codes and complete a drive cycle to verify the fix and ensure the monitor runs

Before you replace the catalytic converter

A downstream oxygen sensor that is lazy or contaminated can mimic a failed catalytic converter, so always test the sensor before condemning the converter.
Exhaust leaks upstream of the downstream oxygen sensor can draw in outside air and cause the sensor to read lean, setting a false P0420.
Engine misfires, rich fuel mixtures, or oil consumption can damage a new catalytic converter quickly, so fix any underlying engine issues first.
Some vehicles have a specific drive cycle that must be completed before the catalyst monitor will run and set the code, so a simple test drive may not be enough.
A P0420 code does not always mean the converter is completely dead; it indicates efficiency has dropped below a threshold, and sometimes a thorough diagnosis reveals a different root cause.

Can I keep driving with P0420?

You can generally continue driving with a P0420 code, but avoid heavy acceleration or towing until it is diagnosed. Prolonged driving with a failing catalytic converter can lead to reduced fuel economy and increased emissions, and in some cases may cause overheating of the converter or damage to other components.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.