DTC P0420: TWC Catalyst Low Efficiency: Description

PCM monitors converter efficiency by comparing the voltage values of O2S1 and HO2S2. Under normal operating conditions, the O2S1 should vary between 10-999 millivolts and HO2S2 should remain relatively steady between 500-800 millivolts. This steady reading of the HO2S2 indicates a correctly functioning catalytic converter. When all parameters have been met, PCM will run this DTC diagnostic at idle. PCM will command rich or lean and monitor the time it takes the HO2S2 to go rich or lean. The longer it takes the HO2S2 to change rich/lean means the converter is storing oxygen and functioning properly. PCM will usually run a 5 second test to determine the efficiency of the converter. If this test fails, PCM may take several tests during several ignition cycles to set this DTC.