Where I used to live, in Commiefornia, smart people would remove their catalytic converter and store it in the garage and replace it with a straight piece of exhaust pipe. Then when they had to get the Commiefornia mandated emissions test done, they would reinstall the catalytic converter for the test. Why go through the bother you ask? Because a catalytic converter can cost up to $1000, and it only takes a few minutes to take it out.