I bought a 2003 Mach 1 with 13k mikes on it. It had a supercharger put on it by previous owner who worked for Ford. I had it emissions tested yesterday, and it failed due to high nox. I’m wondering what the possibilities for cause could be. I don’t pretend to know anything about engines. My husband put regular unleaded in it, and we’re still on that tank, so my first question is, would running through it and getting it back on premium make a difference? The limit is 1.xx, and it tested at 5.xx. Just trying to prepare myself for what I might be spending at the shop if something simple like switching fuel doesn’t work.