Quoted from a muffler shop:
Jeff answered 3 months ago:
"I Run Exhaust Pros in Richfield, MN. the problem is indeed a plugged Cat. Ford had the bright idea of running the primary cats too close to the manifold, so if your car isnt running perfect, you get unburnt fuel in the cat, which eventually ignites and either shatters the ceramic, blowing the pieces into the secondary cat and plugging it there, or melting the ceramic at the primary resulting in a big slag ball. for those of you that talked about soaking and cleaning, dont bother, it wont work. for those that changed out the plugged secondary, the reason you are throwing a code now is that the primary cat is empty. The car will run, but will be running rich ALL the time, making noticeable difference in fuel economy."
Mark