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

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

You just pulled a P0420 from a vehicle indicating the catalyst system efficiency for one of the banks is below threshold. What is next? Could it be just the oxygen sensor? Or, could it be the catalytic converter? These articles will help you understand why the OBDII code was set and what further test must be performed to determine the root of the problem like a coolant leak, oil blow by or other problems with the engine.



 OBDII Codes and Catalytic Converter/Oxygen Sensor Problems 

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

Diagnostic Solutions: Catalytic Converters

“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



Catalytic Converter Diagnosis

The catalytic converter is probably the most important emission control device on a vehicle because it cleans up the pollutants in the exhaust. Thanks to fuel injection, oxygen sensors in the exhaust manifolds, and a feedback fuel control system, emissions are kept to a minimum. Even so, some pollution is still produced and must be eliminated by the converter. So the converter must be in good condition and working correctly to keep tailpipe emissions to an absolute minimum. And if there’s a problem? Then your customer’s vehicle will probably need a new converter.

Converter problems typically fall into one of five categories:… More

 Mischievous Cats (Catalytic Converters)

By definition, a catalytic converter should last the lifetime of the vehicle. It can last this long because a catalyst is something that, just by being present, causes a reaction to other materials without itself being a participant in the reaction. However, real life has proven that the life span of a catalytic converter varies as greatly as the life span of the vehicle itself. One catalytic converter may not fail in 200,000+ miles, while another won’t even make it out of the vehicle’s base warranty. But one thing is for sure, they aren’t going away as long as vehicles are powered by fossil fuels. So let’s talk some about how they work, then move on to spotting one that is misbehaving… More

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