Could be. However, often the O2 sensor failure is not detected by the computer. It can get sluggish. As far as a shotgun approach it is cheaper to try than the cat. Also I think it is more likely. Most cats are replaced because they rattle apart as far as I know. As long as the O2 sensor is fairly cheap I would throw one in. The Bosch ones are kind of spendy but the universal ones are 30.00-45.00. But I don't know how you pick a universal one since I have seen 1 wire, 2 wire, 3 wire and 4 wire... Maybe he could get the O2 sensor tested for a reasonable price?

Mike

Jonathan, those increases are relatively marginal. Could be plenty of factors, most probably minor. If you did a lot of city driving recently, versus a long highway trip, etc.

When I bought my E430 this past April, it scored perfect 0's. Earlier, the previous owner did a test and got some low numbers for HC and CO. So the car actually 'improved' over time, without having any work done to the car.

I wouldn't worry about those numbers at all. You could always do a follow-up test in a few months. If the numbers keep getting worse, you could have an issue somewhere, maybe the cats. Just my $0.03.

can the knock sensor malfunctioning hurt your HC as well?

That's a trick question.
It's ( knock sensor ) job is to retard ignition timing when it " hears " knock.
If it did not work, and the timing was way too far advanced, the HC's would likely be elevated. ;)