If your vehicle has a check engine light on, with a rough running engine, is slow to accelerate, or your car smells like rotten eggs, you will need to diagnose the problem with a scan tool. If the scan tool reads code P0420 or code P0430 “catalyst system efficiency below threshold bank 1 and/or bank 2”, this means the vehicle’s computer has detected an issue with the catalytic converter. Generally, if there is an issue with the catalytic converter, there is a larger issue elsewhere. It could be with leaking coolant, engine oil, or gas. Find out what it means and what to do if a check engine light appears for engine codes P0420 and P0430 with these steps.