My truck has been missing at idle since I got it (another thread going on that) and today my CEL came on-- p0420. I crawled under the vehicle (it seems like I've been lying on the ground underneath it or under the hood more than I've been driving it) and hit it with a mallet, and it did not rattle. It actually doesn't look all that old, and is stamped with 03/08 by the serial numbers. Does this mean that it was manufactured in March? If so, it would be safe to assume that the downstream sensor is bad?

I'd have to replace both of course. This could explain the rough idle. I would hope that a 7 month old cat couldn't have gone bad already.