If it says system too rich, bank one, it's a vacuum elbow with a hole in it. Usually on the left side of the intake manifold. If the engine cooling fan is also doing strange things, the culprit is the ECT sensor, which often starts as a rich code before it tells on itself. Generally an 02 sensor tells on itself very quickly, as do most other sensors. Vague codes are usually vacuum lines. Have only replaced two fuel pressure regulators on 850's, ever.

Hope this helps,
Chris

Are there performance deficits, like poor mileage or hard to start when warm, or are the plugs carbonized? If so, two possibilities are defective fuel pressure regulator or defective/poorly connected engine coolant temp sensor. These would not trigger the check engine light on their own but would cause a very rich mixture. I have had both these happen on different cars.

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