P0420 — Catalyst system efficiency below threshold (bank 1)
What this code generally means, its most common causes, and how serious it is.
TYPE: GENERICSYSTEM: CATALYST AND EMISSIONS
What does the code P0420 mean?
The control module compares the oxygen sensor signals before and after bank 1's catalytic converter. When the two readings look too similar, it means the converter isn't doing enough emissions-reducing work anymore.
Common possible causes
An aging or contaminated catalytic converter— normal wear with mileage.
A failing oxygen sensor (upstream or downstream)— skews the calculation even if the converter is fine.
An exhaust leak near the sensors— outside air that throws off the reading.
An unresolved engine problem that damaged the converter— a misfire or a prolonged rich mixture — this code is then a symptom, not the root cause.
General severity
Generally minor for driving — worth fixing
This code almost never changes everyday driving — the car runs normally. It will fail an emissions test and is worth fixing, especially because it can be a symptom of another problem (a misfire, a too-rich mixture) that's worth resolving before replacing the converter.
⚠️This is still a general severity for this code, not a diagnostic for your car. Only a real read of your codes, with your mileage and vehicle history, can tell what's really going on.
Get a diagnostic for YOUR car
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Frequently asked questions
Should I replace the catalytic converter right away?›
Not necessarily — since this code can be caused by another problem (a misfire, a failing sensor), it's worth checking those first before replacing an expensive part for nothing.
Can I pass inspection with this code?›
No, usually not — an active emissions code will typically fail an emissions test.
Can I keep driving with this code?›
Yes in the vast majority of cases, driving isn't affected. It's mainly an emissions and compliance issue, not an immediate safety one.