There are many ways to test a catalytic converter; one of them is to simply smack the converter with a rubber mallet.
A three-way catalytic converter (TWC). Courtesy of American Honda Motor Co., Inc.
If the converter rattles, it needs to be replaced and there is no need to do other testing. A rattle indicates loose catalyst substrate, which will soon rattle into small pieces. This could be called a test, but it is generally not used to determine if a catalyst is good.
A vacuum gauge can be used to watch engine vacuum while the engine is accelerated. Another way to check for a restricted exhaust or catalyst is to insert a pressure gauge in the exhaust manifold's bore for the O2 sensor.
To measure exhaust system backpressure, insert a pressure gauge into the oxygen sensor's bore in the exhaust. Courtesy of Isuzu Motors Limited.
With the gauge in place, hold the engine's speed at 2,000 rpm and watch the gauge. The desired pressure reading will be less than 1.25 psi (8.61 kPa). A very bad restriction will give a reading of over 2.75 psi (18.96 kPa).
The converter should be checked for its ability to convert CO and HC into CO2 and water. There are three separate tests for doing this. The first method is the delta temperature test. To conduct this test, use a hand-held digital pyrometer. By touching the pyrometer probe to the exhaust pipe just ahead of and just behind the converter (Figure 30 - 21), there should be an increase of at least 100°F (38°C) or 8% above the inlet temperature reading as the exhaust gases pass through the converter. If the outlet temperature is the same or lower, nothing is happening inside the converter. To do its job efficiently, the converter needs a steady supply of oxygen from the air pump. A bad pump, faulty diverter valve or control valve, leaky air connections, or faulty computer control over the air injection system could be preventing the needed oxygen from reaching the converter. If the converter fails this test, check those systems.
The next test is called the O2 storage test and is based on the fact that a good converter stores oxygen. Begin by disabling the air injection system. Once the analyzer and converter are warmed up, hold the engine at 2,000 rpm. Watch the readings on the exhaust analyzer. Once the numbers stop dropping, check the oxygen level on the gas analyzer. The O2 readings should be about 0.5 to 1%. This shows the converter is using most of the available oxygen. It is important to observe the O2 reading as soon as the CO begins to drop. If the converter fails the tests, chances are that it is working poorly or not at all.
This final converter test uses a principle that checks the converter's efficiency. Before beginning this test, make sure the converter is warmed up. Calibrate a four- or five-gas analyzer and insert its probe into the tail pipe. Disable the ignition. Then crank the engine for 9 seconds while pumping the throttle. Watch the readings on the analyzer. The CO2 on fuel injected vehicles should be over 11% and carbureted vehicles should have a reading of over 10%. As soon as you get your readings, reconnect the ignition and start the engine. Do this as quickly as possible to cool off the catalytic converter. If, while the engine is cranking, the HC goes above 1,500 ppm, stop cranking; the converter is not working. Also, stop cranking once the CO2 readings reach 10 or 11%; the converter is good. If the catalytic converter is bad, there will be high HC and, of course, low CO2 at the tailpipe. Do not repeat this test more than one time without running the engine in between. If a catalytic converter is found to be bad, it is replaced.