Many problems with the electrical system. The Engine Control Module has been replaced twice and it still has intermittent electrical failures.
Parking brakes locked up at 34,000 miles.
Motor mount broke at 40,000 miles.
Heater core seals started leaking at 38,000 miles.
Power steering hoses starting leaking at 44,000 miles.
Numerous engine oil leaks. Small, but annoying.
Driver's door has air leaks - annoying wind noise at freeway speeds.
While I enjoy the performance and comfort of this car, it is extremely unreliable.
What you must keep in mind is that 1999 is the first year Volvo made the S80. There are kinks that haven't been worked out. The Turbo version, which you have, often have more problems... the more you have, the more there is to go wrong. Volvo's in general are known for their mechanical reliability overall, but not their electronic. You would be better off with a 2000 or 2001, where the problems have been taken care of.
BTW--I happen to own a non-turbo 1999 Volvo S80, and so far, at 37,000 miles, I have had a failed O2 sensor, and, more importantly, my alternator blew. (350 dollar repair). When you think about it, yes, Volvo's are expensive to take care of, But You will have more of the inexpensive repairs, rather than the 3000 dollar transmission problems. Generally, you can have codes reset in the Volvo S80 by a dealership, if all you have is an error on the message screen, but no other noticeable problems (IE, an O2 sensor computer code needing resetting because of a "Emission System Service Required" error. If you take out the battery and then reconnect it within ten minutes of shutting off your car, this is more than possible.) I hope I have been of SOME help.
Volvo S80 T6 'multiplex electrical system' a disaster (or an experiment that failed) depending if you are the owner or Volvo. Mounting circuit boards on master brake resevoirs that cost $800 to replace is absolute madness. Volvo lost the recipe with the new front wheel drives. No excuse for 'turbo' type issues, Volvo was a pioneer in this area and the twin turbo S80 is a prime example of failed re-engineering. Funny how this engine was tested for over two years in the 960 body by Volvo and performed extremely well. Cost cutters at Volvo were successful in producing a failed product for the consumer and then added insult to injury by failing to stand behind their vehicle. Save your money and grief, buy another brand with reliability and a warranty that means something, too many exclusions in the new Volvo front wheel drive cars.