Check Engine Codes / P0420
P0420 Code
Catalyst System Efficiency Below Threshold (Bank 1)
What it means
The downstream oxygen sensor is seeing the catalytic converter clean up less exhaust than it should. It can mean a worn-out cat — but sensors and exhaust leaks trigger it constantly.
Can you keep driving?
Yes — drive normally, but don't ignore it for months; a truly failing cat can clog.
Common causes
Listed most-common first. Fix cost depends on which one it turns out to be — check your quote free below.
⚠️ Don't get ripped off
The #1 misdiagnosed code in the business. Many shops jump straight to a full catalytic converter without first ruling out a much cheaper oxygen sensor or a simple exhaust leak. Always ask what testing was done on the cat itself before approving the big-ticket fix.
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Related codes
Informational only — codes share symptoms across vehicles, so proper diagnosis on your specific car always comes first.