TruckCodeLookup

Sign in
P0420Cars & SUVs
Moderate

P0420 — Catalyst system efficiency below threshold (Bank 1)

The engine control module (ECM) detects that the three-way catalyst is no longer converting exhaust gases effectively. This fault code is common in cold climates after harsh winters, when short trips keep the catalyst from reaching proper operating temperature.

Ad — development mode

320 × 50 — Mobile

Google AdSense

Likely causes

  • Catalyst worn out or damaged by oil or coolant contamination
  • Upstream/downstream O2 (lambda) sensors faulty or lying
  • Exhaust leak or incorrect air-fuel mixture upstream

Ad — development mode

320 × 50 — Mobile

Google AdSense

Recommended checks

  1. 1

    Check O2 sensor live data with a scan tool before replacing the catalyst

  2. 2

    Fix any exhaust leak or mixture issue (P0171, P0172)

  3. 3

    Replace the catalyst only after a full diagnosis confirms it is bad

Ad — development mode

320 × 50 — Mobile

Google AdSense

Parts to replace