AutoCodes

OBD-II / P0420

Catalyst system efficiency below threshold

The control module compares signals before and after the catalyst and sees less exhaust cleaning than expected. This code alone does not prove the catalyst must be replaced.

Possible causes

  1. 01Exhaust leak
  2. 02Aged or unstable oxygen sensor
  3. 03Degraded catalyst
  4. 04Incorrect mixture or combustion

Safe checks

  • Check related mixture or misfire codes.
  • Inspect for leaks only when the exhaust is cold.
  • Compare sensor data before replacing parts.

P0420 · Ford 21-2381 · 2026-09-25

Worked examples

Documentary case, checked on 25 September 2026: Ford bulletin 21-2381, dated 18 November 2021, discussed P0420/P0430 for 2020–2021 Ford Expedition and Lincoln Navigator vehicles. Ford now marks that source as superseded or deactivated. It is therefore not a current repair instruction for any car displaying P0420. The useful conclusion is to check model, year, VIN, related codes and procedure validity before authorizing parts. Keep freeze-frame data and ask the workshop for the current reference; do not recreate a driving cycle from an archived bulletin.

  1. 01 → Save all codes and freeze-frame data before clearing memory.
  2. 02 → Check related mixture or misfire codes.
  3. 03 → Original editorial content; always confirm manufacturer procedures and specifications.

Source: Ford 21-2381 ↗NHTSA vPIC ↗NHTSA · VIN ↗Print report · TXT ↓

Not a substitute for workshop diagnosis. It helps you understand and decide first. Original editorial content; always confirm manufacturer procedures and specifications.