It certainly could be nothing more than a burned out bulb, although there have been a fairly large number of reported instances where an unscrupulous dealer (or private individual) will simply remove the bulb to make a sale.

The service light should extinguish itself approximately 2 minutes after start-up... it's just a reminder, not a diagnostic tool. And it can be easily reset.

To replace the bulbs requires that the instrument cluster be removed. It sounds and appears more complicated than it actually is.

Look at the following link to see if it's something you'd be interested in tackling...

http://www.seawinds.ws/volvo/instrument_cluster.htm

If you decide to do it, I recommend that you replace all the bulbs while you have the cluster in your hands. And be sure to follow the advice that recommends towels or rags at the top of the dash as you remove the cluster, work slowly and carefully... scratch it, or break a pointer, and you'll spend a whole lotta time kicking yourself.

Good luck

Marty
'96 855T