I have a 1996 855R and I'm having the same problem with one of my O2 sensors. The codes got set the first time last year about thanksgiving, took it in, said it was a bad front O2 sensor, reset the codes (ofcourse the codes got set again before the service), scheduled a service to get it fixed (along with the cat which was just about to go out of warrenty), but when they took a look at the sensor in question they said it looked just fine. After some more digging they found that several (2) vacuum seals on the intake were broken, and apparently this can cause the O2 sensor(s) to set codes indicating their failure. So I got the vacuum seals replaced, and everything was good up untill a couple weeks ago when I noticed the turbo wasn't giving as much boost... As luck would have it I had already scheduled the 80K inspection, so I waited.

Today it turns out that when the guys at seattle volvo started it up, the check engine light came on, and it was the front O2 sensor again... I guess the point to all this rambling is make sure its really the O2 sensor and not a broken vacuum seal, because in my case the codes were getting set every couple weeks until I got the seals replaced and now it's been about 6 months since the check engine light came on. This time I'm sure it's the O2 sensor, because seattle volvo said all the vacuum seals and intake system checked out.

Anyway, I hope this was helpful.