the service lights on your S80 can only be turned off and scanned by the computer at the dealer which I know you didnt want to hear that but they are designed that way to make you go back to the dealer. the only way you can make them go away is by removing the bulbs other wise there is a problem with the stability traction control that is why the stc and pwr steering light is on they are related to the same problem and yes it will probably be expensive to fix and yes it probably is some tiny sensor that failed that is attached to something else that cannot be replaced seperatly so you need to take apart half the car to get to it. Alot of times there was this speed sensor that is in the power steering rack that checked the steering angle that went bad and the entire steering rack had to come out because the part was not available seperately so we had to drop the subframe and replace the entire rack and realign the entire car since the rear wheels could throw off the alignment specs if they were not set as well
Source(s):
Used to work at VOLVO master mechanic for 5 years