A clogged catalytic converter can be caused by a variety of factors, including:
Unbolt the cat, soak it overnight in a degreaser (e.g., Simple Green HD) or a 10:1 water/muriatic acid mix, rinse heavily, dry, reinstall. Review: how to unclog a catalytic converter
The catalytic converter is an emissions control device that reduces pollutants in a vehicle's exhaust system. Over time, it can become clogged with debris, carbon deposits, and other contaminants, leading to decreased performance, fuel efficiency, and potentially causing the vehicle to fail emissions tests. A clogged catalytic converter can be caused by