In Atlanta metro, they plug in to the OBDII port and look for codes. They also check idle speed. When they did mine a month ago, they also looked to make sure the CAT was still there. Their computer links directly to the State, so the only way to "hide" something is to have something to connect to the OBDII plug on their computer and give the no code and idle speed signal (maybe more??). I have heard people fail when they cleared the codes because they did not have the drive cycle completed. I always cleared mine using the screw on the back of the ECU and never had any problems.