DTC P0420: Catalyst System Efficiency Below Threshold: Description

WARNING: This page is about a different car, the 2002 Saturn LW200, 2002 Saturn L200, and 2002 Saturn L100. However, it is still accessible from the selected car via links, so may be relevant.

Three-Way Catalyst (TWC) efficiency is measured by how well it can store oxygen. PCM monitors converter efficiency by comparing voltage values of Heated Oxygen Sensor 1 (HO2S1) and Heated Oxygen Sensor 2 (HO2S2). Under normal operating conditions, HO2S1 should vary between 10 millivolts and 1065 millivolts and HO2S2 should remain relatively steady 500-800 millivolts. This steady reading of HO2S2 indicates a correctly functioning catalyst. When all parameters have been met, PCM will run a 5 second diagnostic at idle. PCM will command rich and monitor time it take HO2S2 to go rich. It will then command lean and monitor the time it takes HO2S2 to go lean. The longer it takes HO2S2 to change rich/lean means converter is storing oxygen and is functioning properly. If 5 second test fails, PCM may take several tests during several ignition cycles to set DTC. DTC sets when PCM has determined that catalyst is no longer efficient.