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Home › Mazda › 2003 › Protege5 Automatic › Repair and Diagnosis › Engine Performance › System › Engine Controls - Self-Diagnostics › Diagnostic Tests › DTC P0421: Catalyst System, Efficiency Below Threshold › Possible Causes

Possible Causes

Warm-up Three Way Catalytic (WU-TWC) converter deterioration or malfunction.
Exhaust system leak.
Loose rear HO2S.
Loose front HO2S.
Front HO2S malfunction.