Huh. I have a 2007. It's been through several sets of tires on 2 different sets of wheels... never had anyone mention a tire pressure sensor. Is TPMS an option or maybe a USA-car thing?

edit: learned something!

http://new.volvocars.com/ownersdocs/.../07xc70_07.htm

page 162

NOTE: If a fault occurs in TPMS, the tire pressure warning light will flash for approximately 1 minute and TIRE PRESS SYST SERVICE REQUIRED will be displayed.