A/C problems are a major issue with 850s. Basically, they leak the
refrigerant. The way to start is a new charge with a dye pack added.
As the new charge leaks out the dye will leave a trace as to the
exact location of the problem (the dye is fluorescent). To do the
job right the refrigerant is removed and a precise measurement of
refrigerant is added back in and a pack of dye is added. In some
cases some lubrication oil is added. My local dealer runs a summer
special where they do this for $99 (talk about your volvo cash cow).

The other way is to buy a can of refrigerant and add it yourself.
This can work or it can cause problems if you add too much (the system
might over-pressure and in that case it opens a valve and vents out
everything and you start over). You have to make the call on
this option.

You an search the archives (below, to the right) on "air conditioning"
and find dozens of threads on the topic.