DTC P0421 & P0431: Catalyst Warm-Up Efficiency Below Threshold: Condition

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0420 is set when catalyst system efficiency is below acceptable threshold on all models except California emission equipped models. DTC P0421 is set when right catalyst efficiency is below acceptable threshold. DTC P0431 is set when left catalyst efficiency is below acceptable threshold. Possible causes are:

Lead in fuel.
Oil contamination.
Cylinder misfire.
Heated Oxygen Sensor (HO2S) malfunction.
Engine Coolant Temperature (ECT) sensor malfunction.
Rear HO2S connector loose.
High fuel pressure.
Exhaust system damage.
Restricted or damaged exhaust tail pipe or muffler.
Catalytic converter malfunctioning.