DTC P0420: Catalyst System Low Efficiency: Description

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

The Powertrain Control Module (PCM) tests the ability of the catalytic converter to lower the 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 the rear Heated Oxygen Sensors (HO2S), to the exhaust leaving the catalyst, read from the post-HO2S. At the appropriate time, the PCM transitions the air/fuel mixture from rich to lean or from lean to rich, depending on the current rich or lean state of the HO2S bank 1 sensor 2. The PCM then initiates a second, opposite transition. The PCM compares the time for the fuel control sensors to cross a calibrated threshold to the time for HO2S bank 1 sensor 2 to cross the same threshold. The longer it takes the HO2S bank 1 sensor 2 to cross the threshold, the higher the oxygen storage capacity. A HO2S bank 1 sensor 2 that crosses the threshold close to the same time as the front and rear HO2 sensors indicates a catalyst with a low oxygen storage capacity, and DTC P0420 will set.