You can try throwing some parts at it but to really diagnose and repair that problem you will need some rather expensive equipment like a Volvo system tester (10K if bought new) and/or a smoke machine, another 1-2K. Otherwise you can try throwing a canister purge valve at it. Depending upon which evaporative system your car runs it is either right ontop of the fan shroud or it is back under the car by the fuel tank and the charcoal canister. Ultimately you could end up putting in a canister, check valve, and a tank pressure sensor. What a nightmare huh? Or you could put the smoke machine on it and find that you had a simple torn vacuum hose somewhere in the system. Evap system diagnosis can be a real pain in the ass for sure and truthfully you might want to just take it to a competent Volov tech with the proper equipment to get it handled and get yourself a guarantee because once he turns that light out it will take a couple of trips to bring that light back on which will put you a week down the road before it comes back on.

Mark