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What Does Code P0420 Mean? Catalytic Converter Efficiency Explained

Written by the ASE-certified technicians at Auto Repair Shops America · 15+ years of hands-on repair experience

The short answer

Code P0420 means the catalytic converter on bank 1 is operating below efficiency threshold, as measured by the downstream oxygen sensor. It's often a worn converter, but exhaust leaks or a bad O2 sensor can trigger it too — test before replacing.

How the catalytic converter is monitored

The engine computer compares upstream and downstream oxygen sensor signals. If the downstream sensor mirrors the upstream one, the converter isn't storing oxygen properly.

Common causes of P0420

Aged catalytic converter, exhaust leaks ahead of the downstream sensor, a lazy O2 sensor, or misfires that damaged the catalyst.

How it's fixed

Repair any misfire first, check for leaks, test sensors, then replace the converter only if confirmed.

Frequently Asked Questions

Can I drive with P0420?

Usually yes short-term, but you'll fail emissions and efficiency may drop.

Do fuel additives fix P0420?

Rarely for long. They may mask symptoms briefly.

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