In order to maintain a reasonably low emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the engine controls system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas. This reaction converts them into harmless water vapor and Carbon Dioxide. The catalyst also reduces NOx, converting it to Nitrogen. The PCM monitors this process using Bank 1 HO2S #2. Bank 1 HO2S #2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the catalysts ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the Bank 1 HO2S #2 signal will be far less active than that produced by Bank 1 HO2S #1. If a problem exists which causes the PCM to detect excessive Bank 1 HO2S #2 activity outside of an acceptable range for an extended period of time, the PCM will set DTC P0420. This indicates that the three-way catalytic converters oxygen storage capacity is below a threshold considered acceptable.

The numbers below refer to the step numbers on the diagnostic table.

4. Before replacing the Three Way Catalytic Converter, correct any conditions which may have damaged the catalyst.