DTC P0421, P0431: Notes

• The PCM compares the number of front HO2S and rear HO2S inversions for a predetermined time. The PCM monitors the rear HO2S inversion counts when the following conditions are met: The PCM detects inversion ratio. If the inversion counts are below threshold, the PCM determines that the catalyst system has deteriorated.
  • The front HO2S inversion counts is as prescribed when the following monitoring conditions are met:
  • The accumulated occurrence time of the following monitoring conditions has exceeded the prescribed time limit:

  MONITORING CONDITION 

  • Engine speed 1,250-2,000 rpm 
  • Calculated load 22-44% 
  • Calculated TWC temperature is 400 °C {752 °F} 

Diagnostic support note 

• This is an intermittent monitor. (CATALYST)
• The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
• PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.
• FREEZE FRAME DATA is available.
• DTC is stored in the PCM memory.

• WU-TWC deterioration or malfunction
• Exhaust gas leakage
• Looseness of HO2S (Front)
• Looseness of HO2S (Rear)
• PCM malfunction