The P0420 is a very common problem on just about every make and model these days. Unfortunately, more times than not the correct repair is going to be to replace the Catalytic Converter.
The Oxygen Sensors measure the amount of emissions in the exhaust going into, and then out of the converter. When they see that the converter is not “cleaning” the exhaust enough to meet the manufacturer’s specs, the Converter is deemed “below threshold” and the P0420 is set. Make sure there are no current cylinder misfires. Make sure there are no leaks in the exhaust system. Which, if there were, you should be able to hear. Sounding like a repeating “tick” or “puff” from the exhaust. Usually more prodominate when the engine is cold. Make sure to include the exhaust manifold when checking for leaks/cracks. If these check ok, then most commonly, the Converter is “on it’s way out” and will need replacement in the near future. Keep in mind, you will need a replacement converter that meets the vehicle manufacturer’s specs. Those “universal” or “discount” converters sold at some after market parts stores do not meet specs, and will not cure a P0420.
On many import model vehicles with 4-cylinder engines, the Converter is mounted just below the exhaust manifold or in some cases as part of the exhaust manifold.
Many older models and domestic vehicles use the under-floor version of the Converter. Most commonly, in-line of the exhaust system.
The Converter converts the three main pollutants from your vehicle’s exhaust: carbon monoxide, unburned hydrocarbon and oxides of nitrogen.