For $24, you can't go wrong with replacing the oxygen sensor. Your local Napa or AutoZone carries the Bosch splice-in-kit replacement O2 sensor for $24. The part number is Bosch #11027.

A dying O2 sensor can still give good valid readings, but cause the car to run rich. As the O2 sensor dies, it starts to show an artificial lean condition and the computer enriches the mixture to bring it back up to what it believes to be 'correct.' Thus, when you take a voltage reading off the 02 sensor, it could be normal, but the car may be running rich to give you this normal reading. I have personally experienced this situation, and for more info, read the following webpage (of Dave Shannons experience):

http://www.volvo2.homestead.com/O2sensor.html

As for life expectancy, depending on your driving and exhaust conditions, you can get as many as 5 or more years out of a Bosch 02 sensor, but I typically replace mine at 3 to 4 just to keep the car running at optimum peak efficiency. I’d avoid purchasing any 3rd party oxygen sensors (like Beck/Arnley), in my experience they seem to die after about 2 years.

If you haven't taken the time to read through Daves webpage, it's definately worth the time. There's lots of good stuff on it and I've learned a lot.

God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 220K