Clogged Catalytic Converter
A clogged catalytic converter results in a loss of power. The catalytic converter is made of an aluminum oxide honeycomb coated with platinum and palladium. These components react to remove CO and HC's from the exhaust stream. A two-way catalytic converter works by oxidizing CO (carbon monoxide) and HC (hydrocarbons "unburned fuel") to CO2 (carbon dioxide) and water. A three-way catalytic converter also removes NOx (oxides of nitrogen) from the exhaust. NOx is formed at high cylinder temperatures. The EGR system is responsible for inhibiting NOx.
When a vehicle is running rich, the catalytic converter is working harder and running hotter than originally 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.
A clogged catalytic converter results in a loss of power. This is because an engine is aspirated and must breathe; if the restriction is severe enough, the engine may not run. The technician in this question uses a vacuum gauge to confirm an exhaust restriction. If the needle on the gauge drops steadily from 2500 rpm, check the exhaust for a restriction. This is probably caused by a clogged catalytic converter.
A back pressure tester is used to confirm a restricted exhaust system. Remove the upstream oxygen sensor and screw in the back pressure tester fitting. Usually the specifications will require back pressure to be below 1.0 psi. at idle, and no more than 4 or 5 psi. at snap throttle. There's a big difference between a late model vehicle and an older model. Always compare results with vehicle manufacturer specifications.