2006 Mazda MX-5 Miata BaseFACTORY OEM SERVICE PROCEDURE

DTC P0421: Notes

DETECTION CONDITION AND POSSIBLE CAUSE (DTC P0421)

DTC P0421 Warm up catalyst system efficiency below threshold
DETECTION CONDITION 
  • PCM compares number of front HO2S and rear HO2S inversions for a predetermined time. PCM monitors number of inversions rear side performs while front side inverts for a specified number of times when the following monitoring conditions are met, PCM detects inversion ratio. If inversion ratio is below threshold, PCM determines that catalyst has deteriorated.

    Monitoring condition 

    • ECT: more than 70°C {158°F} 
    • Calculated TWC temperature: more than 400°C {752°F} 
    • Engine speed: 1,500-3,000 rpm 
    • LOAD: 15-48%  (at engine speed 2,000 rpm  )
    • Time with purge control system does not operate: more than 3 s 
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.
  • DIAGNOSTIC MONITORING TEST RESULTS is available.
  • FREEZE FRAME DATA is available.
  • The DTC is stored in the PCM memory.
POSSIBLE CAUSE 
  • TWC deterioration or malfunction
  • Exhaust gas leakage
  • Loose front HO2S
  • Loose rear HO2S
  • PCM malfunction
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