P0420 OBD-II code — what it means and what to check

ℹ Guidance only — not a substitute for inspection

Enter the trouble codes from your scanner to see what each one means, how urgent it is, and which checks to do first.

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Your trouble codes

Enter the codes exactly as your scanner shows them. Separate several codes with spaces or commas.

A letter followed by four characters, for example P0301. Up to ten codes at a time.
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Your codes, interpreted

What this means
⚠ Book a service. The codes you entered describe faults that get worse, or more expensive, if left.
What to doBook a workshop inspection. Normal driving is usual with these faults, but they should not be left indefinitely.

Your codes

  1. P0420 — Emissions control (service soon). The catalytic converter on bank 1 is not cleaning the exhaust as effectively as it should. The converter itself is a common cause, but so is anything upstream that has been feeding it unburned fuel.

Checks to do first, in order

  1. Fix any misfire, lean or rich codes first — replacing a converter without fixing what killed it will destroy the new one too.
  2. Check for exhaust leaks between the engine and the downstream oxygen sensor.
  3. Have both oxygen sensors' behaviour compared before condemning the converter.

Codes this tool cannot interpret

How this result was reached
  • Interpretations cover standardised codes only, and describe what a code means rather than what is wrong with your specific vehicle.
  • A stored code identifies the circuit or system reporting a fault. It does not identify the failed part — a sensor commonly reports a problem it did not cause.
  • Reviewed by Project Owner on 2026-08-26.
  • This tool covers a reviewed subset of standardised codes. A code it does not recognise is reported as undocumented rather than guessed at.