Hi guys I have a 1999 S80 Volvo that I try to maintain and upkeep most of the easy to intermediate work upon the car, can anybody tell me what sensor this is and how I can connect it back to the car? (The location is under the Air filter box on the driver side)

 

I am not sure what it is, but if your car is operating normally it is probably meant to be disconnected. Volvo uses the same wire harness across many cars so you may find plugs that aren't connected. Someone with more knowledge might be able to let you know what it is though.