Restricted Exhaust Flow
A combustion engine is like an air pump. Air that is drawn into the intake manifold must be able to flow through the exhaust system. Too much resistance caused by a clogged catalytic converter results in a substantial loss of horsepower. A clogged catalytic converter results in exhaust backing up into the engine's combustion chamber.
When an engine's air-fuel ratio is trending rich, the catalytic converter is working harder and running hotter than it was intended. The excessive fuel ignites in the converter instead of the combustion chamber. This heat damages the substrate and causes it to melt, restricting exhaust flow, causing excessive backpressure. The substrate can become loose and rattle. If the converter rattles when tapped with a plastic hammer, it must be replaced.