DTC P0421:00: Notes

• The PCM compares the number of A/F sensor and HO2S inversions for a predetermined time. The PCM monitors the HO2S inversion counts when the following conditions are met: The PCM detects inversion ratio. If the inversion ratio is below threshold, the PCM determine that catalyst system has deteriorated.

• The A/F sensor 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:

• Catalyst converter temperature: more than 400 °C {752 °F} 
• Engine speed: 1, 300-3, 000 RPM (MTX), 1, 200-3, 000 RPM (ATX) 
• - LOAD: 16-48 % (engine speed: 2, 000 RPM) 

Diagnostic support note 

• This is an intermittent monitor (catalyst).
• The MIL illuminates if PCM detects above malfunction condition in two consecutive drive cycles or in one
• drive cycle while DTC for the same malfunction has been stored in PCM.
• PENDING CODE is available if PCM detects the above malfunction condition during first drive cycle.
• FREEZE FRAME DATA (Mode 2)/Snapshot data is available.
• The DTC is stored in the PCM memory.

• Exhaust system leakage
• A/F sensor loose
• HO2S loose
• TWC deterioration or malfunction
• PCM malfunction