Emissions · Fault code

P0420

Catalyst System Efficiency Below Threshold (Bank 1)

The ECU judged the catalytic converter to be storing less oxygen than it expects, by comparing the sensors before and after it. That judgement can be fooled.

Severity EXPERT Low — worth checking, not urgent
Driveability EXPERT Usually safe to drive while you diagnose
Tuning EXPERT Diagnose first — do not tune onto this

Check first — in this order

The order is the product. A list of "possible causes" is what every parts site publishes, and it is why people replace parts blindly.

  1. Compare the upstream and downstream oxygen-sensor traces. A healthy cat makes the downstream signal lazy and flat; a failing one lets it mirror the upstream. This comparison is the whole measurement. EXPERT
  2. Rule out exhaust leaks before and around the cat — unmetered air reaching the downstream sensor reads as low efficiency. EXPERT
  3. Check the downstream sensor itself for ageing and contamination before condemning the converter. EXPERT
  4. Check for an upstream cause that aged the cat — long-term rich running or oil/coolant contamination; fit a new cat into that and it fails too. EXPERT

What this code does not license you to replace

  • That the catalytic converter has failed. A slow downstream sensor or an exhaust leak reads the same.
  • That a new cat fixes it before the sensors, leaks and any upstream cause have been excluded.

Companion codes

Codes whose presence alongside P0420 changes the path.

P0171P0300
No workshop sign-off yet. The diagnostic order on this page has had a technical review of the knowledge model, performed by a model — not a manufacturer certification and not a mechanical inspection of any vehicle. No tuner has yet put their name to this ordering. Treat it as a reasoned starting point, confirm with measurement, and get a professional to sign off before you condemn a part.

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