I have a 1996 850 Volvo. What does the code 172 mean and what repairs are required?

Code P0172 is for system too rich. This can be a problem with on O2 sensor, fuel pressure, leaking injector, cylinder misfire. It will require a little more diagnosing before a specific repair can be determined. It may also be caused by a vacuum leak.

P0172: System Too Rich (Bank 1) - From a quick search, Tech has said this indicates a vacuum leak. Also, see this thread which sounds similar to your issue: http://www.volvoforums.com/m_57903/tm.htm.