Clogged Catalytic Converter
A catalytic converter is made of an aluminum oxide honeycomb coated with materials like platinum and palladium. They react and remove (CO) carbon monoxide and (HC) hydrocarbon. A two-way catalytic converter works by oxidizing CO and HC to (CO2) carbon dioxide and water. A three-way catalytic converter also removes (NOx) oxides of nitrogen from the exhaust.
When a vehicle is running rich, the catalytic converter is working harder and running hotter than intended. This heat can damage the substrate and cause it to melt and degrade. This results in excessive back pressure. Sometimes the substrate will become loose. If the converter rattles when tapped with a plastic hammer, it must be replaced.
Perform a back pressure test to confirm any suspicions of a clogged catalytic converter. Back pressure testers are used to test for restrictions in the exhaust flow. Remove the upstream oxygen sensor and install the tester. The specifications will require back pressure to be below ~1.0 psi. at idle and no more than 4.0 or 5.0 psi. at snap throttle. There's a big difference between a late model and an older model vehicle. Check with the vehicle manufacturer’s specifications to before testing.