DTC P0420: TWC System - Low Efficiency: Description

The Powertrain Control Module (PCM) tests the ability of the catalytic converter to lower exhaust emissions. The PCM determines the performance of the catalytic converter by calculating the oxygen storage capacity. The PCM calculates this data by comparing the oxygen content of the exhaust entering the catalyst, read from the front and rear Heated Oxygen Sensor (HO2S) to that leaving the catalyst, read from the post HO2S.

At the appropriate time, the PCM transitions the air fuel mixture from rich to lean or lean to rich depending on the current state of HO2S bank 1 sensor 2, rich or lean state. Then, the PCM initiates a second, and opposite transition. The PCM compares the amount of time for the fuel control sensors to cross a calibrated threshold to the amount of time for HO2S bank 1 sensor 2 to cross the same threshold. The longer the HO2S bank 1 sensor 2 takes to cross the threshold the higher the oxygen storage capacity. An HO2S bank 1 sensor 2 that crosses the threshold close to the same time as the front and rear HO2S indicates a catalyst with low oxygen storage capacity, and DTC P0420 will set.