Originally Posted by UMD_MaxSE

Installing a Y-pipe, or intermediate pipe, will not cause you to fail emissions testing given that you test the car when it is warmed up and that you have not removed the main cat. The Y-pipe removes the precat that is built into the stock pipe. The purpose of the precat is to cut down on emissions when the car is cold since it takes some time for the larger main cat to warm up to operating temperatures. So, if the precat is removed, the only time you may possibly fail emissions is when the car is cold. If the car is running at normal operating temperatures and you have your main cat in place, you should not fail emissions testing. If a visual inspection is involved and the inspector knows what to look for, then that changes the story