P0420 Catalyst System Efficiency Below Threshold (Bank 1)
The oxygen sensor after the catalytic converter is seeing exhaust that's too similar to the exhaust going in – so the car's computer has decided the converter isn't cleaning the exhaust as well as it should.
Common causes
- Worn or failed catalytic converter
- Faulty downstream (or upstream) oxygen sensor
- Exhaust leak near the sensors
- An engine problem such as a misfire or rich running that damaged the converter
Symptoms
- Usually none besides the check engine light
- Failed emissions test
- Sometimes a rotten-egg smell or reduced power if the converter is clogged
Can I drive with it?
Usually safe to drive in the short term. Fix any misfire first – unburned fuel can overheat and ruin a new converter.
Tip: Check for other codes first. Replacing a converter without fixing the misfire or fuel problem that killed it usually means a second P0420.
Which scanner reads it?
Any OBD2 scanner reads it. A tool that graphs live data lets you compare the two oxygen sensors, which helps tell a bad sensor from a bad converter.
Budget tools like the ANCEL AD310 read it; tools with live data and graphing – like the BlueDriver or OBDLink MX+ – help you find the cause. Not sure which fits you? Try the Find My Scanner quiz.
In the data
How common is it? At New York’s emissions inspections in 2020–21, P0420 was the #1 code found on cars that failed – 6.0% of all the codes found.
Of the vehicle complaints owners filed with NHTSA from 2015 to 2025, 245 quote P0420 – most often from Kia, Chevrolet, Ford owners. Complaints lean toward serious problems, so treat this as a sign of how often the code turns up in real reports, not how often cars get it.
Both sources are compared on the Trends page.
Related codes
General information to help you understand a code – not a diagnosis. Your car’s service information or a qualified mechanic has the final word. See how we research.