[Gohan Ryu]

Heat up the catalytic converter right before you test. Take it for a long highway run to make everything hot as it can be, and keep revving the engine while you are waiting to test to keep it hot (if the guy doing the test will let you rev up in front of the testing station - heating up the cat isn't allowed and if an inspector caught him allowing people to rev up he'd probably be fined).

The hotter the cat is the cleaner your exhaust will burn.