Volvo Repair: O2 Sensor Replacement, catalytic convertor, o2 sensors

Question
I've got a 1998 S70GLT; in a recent test, my mechanic analyzed my fuel mixture and suggested it was running high because the O2 sensor needed to be replaced.  The car has about 147K miles on it, by the way.

The question is:  Apparently there are two O2 sensors.  Which of them would I need to replace or would it make most sense to replace both of them from a preventative maintenance perspective?

Thanks,
-roy

Answer
First, I'm sorry for the delay getting to your question.  I have been orlando for the past few days for XC90 V8 traingin and have been unable to take emails.

The front sensor is the one you need to switch.  That one analyzes engine mixture, and the second one is used to analyze the mixture after it flows through the catalytic convertor to make sure it is sufficiently cleaning the exhaust gases.  The rear one RARELY fails so you should just leave it.  They can be expensive too so don't waste your time.  Shouldn't take you very long as you are able to get to it comfortably.  It may be hard depending on what tools you have and where you are doing it.  Hope that helps.

James