If you get a chance, get us a picture of the down pipes. The fact you are getting a code, I'm guessing from converters operating outside of threshold, tells me you probably have high flow/race style catalytic converters or no cat's at all.
If that's the case, there is nothing the dealer can do to fix the CEL short of replacing the down pipes with factory equipment.
Do you have emission testing where you live? If not, your best bet would be to get a tune and have the rear O2's turned off. If you do have testing, you should get in touch with the original owner, maybe through the dealership, and get a hold of the factory down pipes.