I have a 2001 S40 with 102,330 miles. I do not know what this gauge is called but normally when it is cold outside, I like to warm my car up. I was told that the hand on this gauge is suppose to be in the middle, it has a 2 red lines at the top, if it reaches to that point, it means the engine is running hot.For some reason, it rarely reaches to the middle, when it does, it will stay for a while, then goes back down again, then my heater starts blowing cool air. It just started within the pass 3 weeks. I do not know what the problem is. Please Help

 

Get it checked out right away. Your cooling system is having a problem. It could be a stuck thermostat, a dirty cooling system, or the waterpump could be having issues too.
You don't want to drive around alot like that.

 

The gauge you are referring to is the temperature gauge which typically is at the half way point when the car is operating in a normal manner. It sounds as though you may have a bad thermostat which is rather easy to change and cheap when you do it yourself. Open the hood and look for the black radiator hose that connects the top of the radiator to the top of the engine (just slightly left of center). The point at which it connects to the motor you will see a metal pipe that is held with two bolts to the engine. When you remove those two bolts off will come the metal pipe and underneath you will see a thermostat. Don't do this when the engine is hot or you will be in danger of scalding yourself.