Catalyst performance below threshold



P0421 - rear (RH) bank manifold catalyst

P0430 - front (LH) bank manifold catalyst

P0431 - front (LH) bank manifold catalyst


Emissions Equipment Information

Three o2 sensor system (KL engine with one catalytic converter) - A front sensor is positioned on the LH bank and a middle sensor is positioned on the RH bank upstream of the catalyst for closed loop fuel control.  A rear sensor is mounted downstream of the catalyst to monitor its efficiency.

Four o2 sensor system (KJ / KL engine with three catalytic converters) - Front sensors are mounted upstream of the RH and LH warm-up three-way catalyst (WU-TWC) for closed loop fuel control.  Rear sensors are mounted downstream of the WU-TWCs to monitor their efficiency.


Detection Conditions

Diagnostic Support Notes

Possible Causes

Additional Information

See for information on TSB Item Number 01-003/01 regarding overly sensitive rear o2 sensors erroneously storing these codes on some Miller Cycle engine equipped models.


[P0420 / P0421 / P0430 / P0431 - From 1998 and 2002 Mazda Millenia factory service manuals]