Code P0420: The Dreaded Diagnostics of Catalytic Converter Replacement and Oxygen Sensors

Before replacing an OBD II converter, always check the diagnostic memory for converter-related DTCs, such as a P0420. Also, make sure that the vehicle has exceeded its emissions warranty. If not, the vehicle must be referred to the closest dealership for warranty services.

OBD II monitors include the catalyst heater, catalytic converter efficiency, secondary AIR, O2 sensor heaters, EGR system, PCV system, thermostat and A/C system (where used). These are all “non-continuous” monitors and are not set until certain driving conditions have been met. The converter efficiency monitor, in particular, is a hard one to set and may require driving the vehicle at various speeds and loads so the OBD II system can get a good look at what’s going on. More

“Three-way” catalysts (TWC) were designed and introduced during the 1980s, not only to oxidize CO and HC, but to use a third chemical process called reduction to break down nitrogen oxides (NOx) into their component elements of nitrogen and oxygen. The oxygen released by reduction would then combine with CO and HC to form CO2 and H2O. In brief, three-way converters can be configured to allow air to be injected upstream during warmup. During normal operation, air is injected downstream between the three-way and the oxidation catalysts to help complete oxidation of CO and HC. The downstream air allows the TWC or oxidation/reduction to operate with a stoichiometric or chemically correct 14.7:1 air/fuel ratio, while supplying the oxidizing catalyst with the extra air needed to reduce HC and CO. In more modern applications, upstream and downstream air injection has been eliminated altogether... More