94 C280 O2 sensor?

Mom's 1994 Mercedes C280 (only about 40K miles on it). Check engine light has been on and recently it failed the state emissions test (just barely on hydrocarbons, mostly on CO levels).

I haven't had the diagnostic code read (yet). If I can't find someone to do this I was thinking of replacing the O2 sensor on speculation as the likely cause. But of course there are two on this car, before and after the cat converter. If I do in fact go this route I can replace one at a time. Which would be the more likely culprit (if it's even one or the other)?

Also, does curing the problem make the check engine light go out or does that only happen with a reset of some sort?

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Steve Kraus
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