The most common cause of an OBD-II code of P0420 is an old or malfunctioning catalytic converter. Other possible problems include exhaust leaks, a damaged oxygen sensor or damaged exhaust pipes. In general terms, a P0420 code indicates that the catalyst system efficiency is below its preferred threshold.
Besides catalytic converter issues, malfunctioning engine sensors can also trigger a P0420 code if too much, or too little, fuel enters into the engine. If the P0420 code is present with other OBD-II codes, the underlying causes of the other codes could be triggering the P0420 code as well. Car owners should address the P0420 code last to see whether fixing the other problems clears it.