Maybe it's time for a new O2 sensor?
 

I'm thinking about replacing my O2 sensor but should I?

About a month ago the check engine light came on and two codes came up, #2 and #26 or #27, can't remember. Anyway #2 is a bad O2 sensor.

Right after the light came on I reset it and it hasn't been on since but since my car has almost 160k miles on it and I don't think it's been replaced should I just go for it?

I've also got my smog test coming up and I'm afraid that if it is bad then I'll probally fail for sure.

The car runs good and the gas mileage has slightly decreased but it's still averages 20mpg, but it could be my driving habits.

...for the money it's worth it
 

I recently failed miserably on my IL emissions test. Knew the idle was a bit rough, which turned out to be the O2 sensor. Changed out the related carboned up plugs, oil change. Passed second test fine.

You didn't mention your model, but on a W126 I can say that for the $25 spent on a universal Bosch, carefully spliced into place, the money and half hour spent makes the decision easy.

I'd say replace, and pull a plug or two to check that they didn't take on too much unburnt fuel if the sensor was bad. I do a lot of city driving so they were a little blacked up. Again, for 20mins and $20 an easy decision.

I seem to always forget to list my model, I'll have to put it in my sig.:p It's a '92 400E.

If I can find one for the cheap I'll get it, but the prices I've seen have been around $80.