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Home › Mazda › 2003 › MX-5 Miata Base, Standard › Repair and Diagnosis › Engine Performance › System › Engine Controls - Self-Diagnostics › Diagnostic Tests › DTC P0420: Catalyst System Efficiency Below Threshold › Possible Causes

Possible Causes

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