"Due to the combination Catalytic Converter/Exhaust Manifold, or Mani-Cat, you are unable (technicalities aside) to pass emissions with an aftermarket header installed"
I will add a bit more detail here if permissible as this thread is about header/cat and the attendant legality's.
The issue with replacing the stock mani-cat as far as the EPA is concerned is this. Start-up emissions. Any car emits more hazardous emissions during (cold) start-up than during several hours of running when the cat is up to operating temp. Hence, Ford engineered the primary cat (the mani-cat) to be as close to the exhaust ports as possible to enhance the heat-up of the cat.
I talked with my local EPA guy about running a wrapped header to a HF cat and this initial 'heat-up' was the primary concern. But! they waffled a bit, and as there were no supporting facts they went back to the letter of the law (EPA doc 20460).
This means that if you(we?) can prove that a wrapped header with a EPA certified HF cat meets or exceeds the OEM spec for emissions during a cold start then there is a very good chance that you(we) could get an exemption. Note: After a cat is warmed up, there is no issue as they all 'do the job' pretty much the same.
An example of such a cat would be a cat that has a pre-heater (Tom mentioned them in a prior thread). I have not been able to find any such cat in numerous searches as they seem to have been replaced by non-powered 'racing cats'.
So if you have a good head for this stuff (or maybe just the tools) it is quite possible that a header/cat combo could be EPA legal.
Note: An exemption is not a CARB certification. But you would not need that unless you intended to sell a header/cat combo.
Steeda SRI w/CAI intake. Trubendz SS 2.5" . FR Susp/Struts. Tune by Tom (FocusPower). Rota Grids. SAP rear spoiler. Custom front air dam, rocker panel bits and Rear valance.
Last of the Hatches #28