As per the title of your question, 3-1-5 means EVAP emmission control system fault. On my 1994 850 turbo, I tracked the problem down to a faulty solenoid valve for the purge control. It COULD have been a vacuum leak in the same system, a faulty (ie - fuel saturated) EVAP charcoal canister, or a faulty solenoid. I had checked alll the vacuum lines (OK), and dremel-cut the charcoal canister open (NOT saturated by fuel vapours, but I replaced the activated charcoal anyway (see your local aquarium dealer) and reassembled and sealed the unit), then finally checked the solenoid. If you follow the vacuum line up from the charcoal cannister (left side of the car, ahead of the front wheel well, behind the bumper) to the area near the top/drivers side of the radiator, you will find the solenoid sheltered inside a cylindrical foam shell. When I checked mine out, it would allow flow through (as checked by blowing and sucking on the ports), even when the engine was cold. It should only allow flow, thus purging the cannister with engine vacuum, when the engine is at the appropriate temperature, and the signal comes from the ECU. My solenoid was stuck in a partially-open position, this giving the error code 3-1-5. The part was sightly over $200 Canadian (Calgary Volvo dealer).

I do not suggest that your's needs the solenoid. This is what fixed mine when the same code appeared. Good luck with yours.
Doug