one thing u can try and this is just for the test,, take off a cpl vacuum lines going to the engine,, this will induce more air into the mixture and make the vehicle burn lean (not as rich),, so there will be less gas fumes from the tailpipe,, this is not good for the motor but itll get u passed,, only other thing u can do is add an air pump to inject air into the exhaust system,, but this is assuming that everything is good on the vehicle and the cat is working properly etc,,,,Art