Have you checked/replaced your EGR? 'Coz that "might" be the problem. Not 100% sure but it's worth a check. And that's part of the emission.
Play hard, Rock harder.
2002 Mustang V6, AT
K&N Air Intake/Tokico HP Series Shocks & Struts
Mach 1 Chin Spoiler/Fog Lights
Dual Flowmaster 40 Series Delta Flow
(That's all folks!)