DTC P0421: Warm-Up Catalyst - Low Efficiency - Bank 1: Description

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

Catalytic converter efficiency is measured by how well the converter can store and release oxygen needed to convert CO, HC and NOx gases to CO2 and water. Under normal operating conditions except heavy acceleration, cold engine temperatures and extended decelerations, the Heated Oxygen Sensor (HO2S) bank 1 sensor 2 voltage should remain relatively steady between 500 mV and 800 mV. This steady reading of the HO2S bank 1 sensor 2 indicates a correctly functioning pre-catalytic converter as the air/fuel ratio changes slightly around 14.7 to 1. If the pre-converter is storing and releasing oxygen correctly, the HO2S bank 1 sensor 2 should hold its voltage value for a certain length of time as the air/fuel ratio increases or decreases. Under relatively steady cruise, the ECM will run a test which records the voltage amplitude values of the HO2S bank 1 sensor 2 as an oscillating air/fuel ratio is introduced. The ECM will then compare the amplitudes it obtained to modeled values of a new-failed catalyst. The closer the actual amplitudes of the HO2S bank 1 sensor 2 are to the modeled near-failed catalyst values, the more inefficient the pre-converter is becoming. DTC P0421 sets when the ECM has determined that the bank 1 pre-catalytic converter is no longer efficient.