I agree that CO may be leaking from a cracked manifold or just from the tailpipe, but either way it is entering the vehicle via the many holes and seams that Ford left unsealed around the rear end and underbody.
if the CO leak is at the manifold prior to the catalytic converter, the CO level is typically 100 times higher (from 20,000 to 50,000ppm, or 2-5%), compared to under 200ppm when the catalytic converter is working properly. But depending on conditions (worse under cold start, for example), untreated exhaust can have over 1000x more CO.