Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.

P0420: Catalyst system efficiency below threshold, Bank 1

The control module believes the Bank 1 catalytic converter is storing and treating oxygen less effectively than its calibrated threshold.

What P0420 means

The control module believes the Bank 1 catalytic converter is storing and treating oxygen less effectively than its calibrated threshold. Bank 1 contains cylinder 1; an inline engine normally has only Bank 1. The code does not prove that the converter itself must be replaced.

Can I still drive?

If the warning is steady and the car runs normally, limited gentle driving is generally possible while arranging diagnosis. Stop and recover it for a flashing warning or active misfire, severe power loss, overheating, smoke, a strong raw-fuel/burning or sulphur smell, or rattling with poor performance. Those signs can accompany converter overheating or restriction.

Symptoms

  • Frequently only a steady engine-management light.
  • Emissions-test failure.
  • Reduced fuel economy or performance.
  • Sulphur-like smell or a rattle from the converter.
  • Major power loss if the substrate has collapsed or become restricted.

Common causes

  • Aged, overheated, contaminated or damaged Bank 1 converter.
  • Exhaust leak near the manifold, converter or lambda-sensor joints.
  • Slow, biased or faulty upstream/downstream oxygen sensor, heater or wiring.
  • Unresolved misfire or rich/lean fuelling fault.
  • Leaking injector or inaccurate airflow/fuelling input.
  • Oil or coolant entering the exhaust and contaminating the catalyst.
  • Unsuitable replacement converter or vehicle-specific software issue.

Safe first checks

  • Retain every code and the freeze-frame data. Resolve active misfire, fuel-trim, temperature and lambda-sensor codes before judging the converter. Once the exhaust is completely cold, look for soot around accessible joints and heat damage to sensor wiring. Note oil/coolant loss, previous misfires and replacement-converter history. Proper diagnosis compares upstream and downstream sensor signals and converter performance; fitting a rear sensor speculatively is not a test.

Typical UK fix cost

Allow £100–£220 for diagnosis. An exhaust leak, lambda sensor or wiring repair may total £180–£500. A suitable aftermarket Bank 1 converter commonly brings the total to roughly £550–£1,800; close-coupled, manifold-integrated or precious-metal-heavy units can be £1,200–£3,000+. Any root-cause engine repair is extra.

Related models

This is a generic OBD-II code used across many makes and models. The exact test thresholds, component layout and repair route vary by engine and manufacturer, so check vehicle-specific technical information before ordering parts.

Related fault codes

Sources

A diagnostic trouble code identifies a system or circuit to investigate. It does not prove that a named part has failed. Preserve freeze-frame data and seek a vehicle-specific diagnosis before clearing the code.