DTC P0421: Catalyst System, Efficiency Below Threshold: Condition

WARNING: This page is about a different car, the 2002 Mazda Protege5 and 2002 Mazda Protege. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when value from rear HO2S becomes closer to value from front HO2S. This is monitored when engine speed is 1500-3000 RPM, vehicle speed is 17.3-74.5 MPH and vehicle is under 15-48 percent load. This is an intermittent monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:

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