OBDII systems with faulty catalytic converters will normally store a diagnostic trouble code (DTC) when the converter begins to fail. The OBDII converters use an upstream and downstream oxygen sensor to measure the differences in oxygen content between the inlet and outlet gases. In virtually every case, the OBDII system will detect a converter failure long before it can be detected using shop equipment.

Consequently, before replacing an OBDII converter, always check the diagnostic memory for converter-related DTCs, such as a P0420.

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