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 or unburned fuel) to carbon dioxide and water. A three-way catalytic converter also removes oxides of nitrogen NOx 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 it was intended. This heat can damage the substrate and cause it to melt and degrade. The result can be excessive back pressure caused by a clogged or plugged catalytic converter. Sometimes the substrate will become loose. If the converter rattles when tapped with a plastic hammer it must be replaced.

An easy way to test a catalytic converter for a rich fuel condition is to use a pyrometer to compare the inlet and outlet temperatures. A pyrometer is a handheld infrared thermometer used to measure temperature from a distance. At normal operating temperature the outlet temperature should always be more than the inlet. If this temperature is in excess of 200°F the engine is likely running rich. Late model catalytic converters have less temperature difference between the two.

Symptoms of a clogged catalytic converter include a loss of power and the vehicle will be extremely sluggish because of the restricted exhaust flow. The technician in this question uses a vacuum gauge to confirm an exhaust restriction. If the needle on the gauge drops steadily at 2500 RPM check the exhaust for a restriction. This will likely be a clogged catalytic converter.

Use can also use a back pressure tester to confirm a restricted exhaust system. Remove an 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 check with vehicle manufactures specifications.