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.

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