Replace your thermostat. It's stuck open. An engine that does not reach operating temp uses more fuel, pollutes more, and wear can be accelerated. Not to mention a heater that just doesn't quite make it. You will need a good male torx bit socket, size 40. Best to get one with at seast a 4 inch long bit. The back bolt on the thermostat housing is hard to reach with a standard length tool. A short socket will not enter the screw straight, and you may strip the screw. Standard temp t/stat is 87 deg. C. Use a Volvo t/stat, and replace every 30 or 40k. If summers in your area are mild, you can use a 92C. stat. Better heat.