Thermostat and sensor are different parts.

Thermostats fail often at around 70K miles.

My thermostat in my 98 S70 T5M has failed at 57K miles
and I see the same low temp gage reading as you do. I plan
to order a new one from volvoparts.com (they are not expenisve)
and follow the instructions at:

http://volvospeed.com/Repair/thermect.htm

Apparently, the T40 Torx screw is the tricky part so I already
purchased the tools recommended at the above page. The job
looks quick so if a mechanic did it I would expect the labor
would not be too expensive.

Have a look at http://volvospeed.com, it is a great resource.