Ever fail emissions with a mac (cat) h-pipe?

[Ry95GT]

Im pretty much anal when it comes to keeping the car tuned, always doing everything on time. Oil was just changed 3 weeks ago. I always use premimun gass. Plugs (Motorcraft) are about 1 1/2 years old with about 5k on them. They are gaped at whatever the factory recomendation was. I havent gotten a check engine light so I dont think its the oxygen sensors. Im gunna try and retard the timming alittle more and go for a longer drive before I take it to the test. I went from work to the emissions place wich was about a 8 min drive and sat idling for about 15 min before they tested it.