I am guessing that you have a pre-cat O2 sensor and an after-cat O2 sensor. It sounds as if your after-cat O2 sensor isn't seeing the results that it expects. That could either be because your cat is bad or your O2 sensor is bad. There are probably a couple of other reasons as well though. You can do an O2 sensor test and monitor the O2 sensors with the proper OBD-II tester. That would tell you if your O2 sensors are operating properly. If your after-cat sensor is locked up at some voltage then I would suspect your O2 sensor is bad. It should always be able to adjust its output somewhat based on the information it is receiving. If the cat is bad, I don't know what other symptoms it would exhibit besides being plugged up. If it is plugged up you can have an exhaust shop install a pressure gauge before and after the cat and determine the pressure drop across the cat. If it is too high, then your cat is plugged and needs to be replaced. As the previous post said if you need to purchase a new cat, a high-flow, aftermarket one would be much cheaper than going directly to Ford.
