DTC P0421: Catalyst Warm-Up Efficiency, Low: 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 is set when catalyst system 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.