P0420: Catalyst System Efficiency Below Threshold (Bank 1)

Common causes

• A worn or failing catalytic converter
• A faulty downstream (or upstream) oxygen sensor
• An exhaust leak near the sensors
• Engine problems such as misfires or running rich, which can damage the converter

Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.

Symptoms

• Check engine light, often with no other symptoms
• A failed emissions or inspection test
• Sometimes a rotten-egg smell or reduced power

Is it safe to drive?

Usually drivable in the short term, but get it checked: if a misfire or rich condition is causing it, the converter can be damaged further.

How to check it with a scanner

1. Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
2. Scan for other codes first. Misfire or fuel trim codes should be fixed before replacing a converter.
3. With live data, compare the upstream and downstream O2 sensors. The downstream sensor normally stays fairly steady when the converter is working.
4. Inspect for exhaust leaks ahead of the downstream sensor.
5. After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.

A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.

Questions

Related codes

• P0430: Catalytic converter efficiency, bank 2
• P0133: Upstream oxygen sensor slow, bank 1
• P0300: Random or multiple cylinder misfire
• All trouble codes
• How to choose a car diagnostic scanner