If the temperature is reading low, or if the engine is taking longer than normal to get up to operating temperature, then your thermostat is sticking open and should be replaced, along with the coolant (if it hasn't been changed in the last 2 years or 30,000 miles).
If the engine is running poorly or your having starting/stalling issues, then the coolant temperature sensor or wiring may be defective.
If the coolant level is very low, this fault could occur, then you may have a coolant leak that will need to be addressed (Replace the thermostat too)
It can be the coolant temperature sensor going bad. I have a 1998 S70 GLT that had this and was replaced twice. If have the coolant temperature sensor replaced you can also replace the thermostat at the same time since that are in the same area of the cooling system.