My GF has a 2001 s40T, and the check engine light is on. I was wondering if i can restart the obd2 computer by unplugging the battery for 30 min or so? I am use to dealing with odb1 computers.

 

If you have an AutoZone or similiar shop nearby, they let you use their ODBII scan tool to check and reset codes. My wife's 00 S40 needed to have the gas cap replaced and that kept the service light coming on. Unhooking the battery will clear the codes, but then you won't know what the problem is.