Just failed emissions test. Bad O2 sensor?

My mom's c220 just failed the emmissions test (I should have known it was going to because of the "check engine light").

My best guess is that one of the O2 sensors is busted, but I'm not sure which one. When I went around to get it OBDII scanned, and all the shops wanted 65 bucks just to "diagnose it". I can't believe they want $65 for a 4 min job.

Anyway, the car has 40k miles on it. Is it normal for an O2 sensor to bite the dust this soon? I replaced the one in my jeep after 100k. Also, my jeep only has one sensor (before the cat).

Why are there 2 sensors in the c220? Can I assume that the precat is the one I need to change b/c its the one that controls the fuel mixture? (I'm fairly certain the sensor is stuck open and causing the engine to run rich)

Can it be anything other than the O2 sensor?

TIA!

Just had another idea- is it the same part# for both O2 sensors?
Maybe I could replace one and if the light stays on, just switch it.