P0421 - OBD-II Trouble Code

Our automotive experts have put together the following information about the P0421 diagnostic trouble code. We've provided the common symptoms that occur when this code is set along with the frequent repairs that address the issues related to the P0421 code.

OBD II Fault Code

  • OBD II P0421

Fault Code Definition
Code P0421 is triggered when the Bank 1 Warm Up Catalytic Converter's emissions reducing efficiency has fallen below the minimum allowable threshold. 


  • Check Engine Light will illuminate
  • In many cases, no abnormal symptoms may be noticed
  • In some cases, there may be a noticeable lack of engine power

Common Problems That Trigger the P0421 Code

  • Defective Warm Up Catalytic Converter
  • Engine misfires have damaged the Catalytic Converter

Common Misdiagnoses

  • Oxygen Sensor(s) are replaced when the real problem is a damaged Warm Up Catalyst
  • Warm Up Catalyst is replaced when the real problem is internal engine damage, which is producing elevated emissions levels
  • Warm Up Catalyst is replaced when a misfiring engine is the real problem

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