What kills O2 sensors? Running rich. What causes a rich condition? Lots of little things:

Short trips with a cold engine.
Air leaks into the intake manifold from vacuum lines that fall off or crack.
A bad MAF.
Engine coolant temp sensor reading too low.
Short trips with a cold engine.

You need to have your engine tested for vacuum leaks. And then have a monitor plugged into the diagnostic port, checking the O2 sensors, engine temps, etc.

If you only drive a few miles to/from work, do it in 2nd gear. That gets the engine warm, oil and coolant, and allows the valves to spin like the engineers intended.

Klaus
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Two great Volvos, a 1967 220 and a 1998 V70R

I wouldn't go and replace that just yet. For starters clear and make sure it comes back...

It could be caused by a misfire, could be air leaking into the system (either side..), fuel pressure could be off, compression could be off, and finally it could be the cat.

I believe this code gets set when the reading on the front is very similar to the reading on the rear O2 sensor. When the Cat is not working they'll be the same, but before you spend a grand or you might as well make sure that's the problem..

I don't know if you've cleared yet or not, but clearing and making sure it comes back, as well as making sure the car runs all the tests before you replace is important. Misfiring would recognize as a separate code, but uneven compression wouldn't (i don't think at least..).