no one does visual or sniff tests in MA, both my 95 and 98 Nissans did not have cats and both did not smell and both did not trigger CEL. With S2000 test pipes that are sold also pass all inspections and only trigger CEL in some cases which can be fixed easy. Moreover, state inspections here are done by mechanics for $30 bucks, no one is going to spend time sniffing or looking at your car, thats why inspection computer does all emission tests with OBD and/or probe that goes inside your exhaust pipe. If probe detects unacceptable emission levels, you dont pass inspection.
and here is HOW CAT WORKS not from a 9 year old. Although its a forum, not a fucking college chemistry exam
"The catalytic converter does a great job at reducing the pollution, but it can still be improved substantially. One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.
One simple solution to this problem is to move the catalytic converter closer to the engine. This means that hotter exhaust gases reach the converter and it heats up faster, but this may also reduce the life of the converter by exposing it to extremely high temperatures. Most carmakers position the converter under the front passenger seat, far enough from the engine to keep the temperature down to levels that will not harm it.
Preheating the catalytic converter is a good way to reduce emissions. The easiest way to preheat the converter is to use electric resistance heaters. Unfortunately, the 12-volt electrical systems on most cars don't provide enough energy or power to heat the catalytic converter fast enough. Most people would not wait several minutes for the catalytic converter to heat up before starting their car. Hybrid cars that have big, high-voltage battery packs can provide enough power to heat up the catalytic converter very quickly."