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

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

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.

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Chris
2007 E550 4Matic - 61,000 Km - Iridium Silver, black leather, Sport package, Premium 2 package
2007 GL450 4Matic - 62,000 Km - Obsidian Black Metallic, black leather, all options
1998 E430 - sold
1989 300E - 333,000 Km - sold
1977 280E - sold
1971 250 - retired

"And a frign hat. They gave me a hat at the annual benefits meeting. I said. how does this benefit me. I dont have anything from the company.. So they gave me a hat." - TheDon

can the knock sensor malfunctioning hurt your HC as well?

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My Cars
Mercedes CLS
Mercedes C-Class W203 - All time favorite

R.I.P.
Mercedes C-Class - W202 Lemon
Mercedes ML - Warrior
VW Jetta – Worst Car I ever owned - Alot of problems

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.

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2007 C 230 Sport.