Couple of things to look at. Your #1, "pre cat" O2 sensor is a prime suspect. These usually fail gradually and will eventually set their own fault code. Your mpg will also gradually fall off and you'll see soot around and inside the tailpipe and maybe see a puff of black smoke when you goose the engine and smell the fumey exhaust. Volvo recommends changing the o2 sensors at 100,000 miles. You may also have a vacuum leak in a number of places. This leans the fuel mixture and a good 02 sensor will richen the mixture to try and compensate. Check volvospeed.com (Bay 13) for common vacuum leak tracing. Just went thru this "too rich" code stuff with a 96 850. Had a vacuum leak and a bad 02 sensor.
Good luck!
bl
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