You might want to try a new bank 1 (firewall side) downstream (I think, could use some help here) oxygen sensor. I'm making the assumption you have a duratec. If that doesn't work, you are still under the 80,000mi emissions warranty to get that pre-cat replaced. Don't know how your aftermarket stuff might affect the warranty though. Also, if the custom exhaust is new and this is the first time this has popped up, you should consider checking for damage to the rear exhaust manifold and pre-cat.
