The most typical code is 4-3-5 for "front sensor slow to respond." I replaced mine about 2 months ago after it throwing enough codes to trigger the CEL a couple of times. I bought the Bosch OEM replacement for about $120 online and replaced it in about 45minutes.

Being a turbo, space is tight near the connector and that's what took the most time. You won't need any special tools to remove or install the sensor. I used an open end wrench.

The new a dab of anti-seize compound on it and all you have to do is screw it in, run it's wire and connector up to the connector on the harness and you're set.

After doing that I replaced the plugs, cap and rotor and saw my average mileage go up from 20mpg to just under 22mpg for my daily commute.

Some guys have cleaned them and gotten more life but I'd say just replace it. Also, I've read here that there is a chance your cat could be damaged by leaving a bad 02 sensor in. I think Bosch (and Volvo?) reccommend replacing every 30,000 miles. Same interval thermostat if I recall.

Good luck.
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Erwin in Memphis, '95 855t