Okay first question...When replacing thermostat, do you have to replace coolant at the same time or can you just replace the thermostat seperately...I replaced my coolant a few months ago, but for whatever reason I reluctantly didnt replace the thermostat.

also, on a phase2(2004) s40, is the temperature reading thats found on the dash related to the thermostat/coolant system or is it solely a reading of the outside temperature..I'm asking this because as stated above, my thermostat is due for replacement, and lately my temperature reading will start at around 68 degrees for example and will rise up to the 80's even though I know its not that hot outside and then the temperature will go back down to where it should be...

 

Drain sufficient coolant into a bucket to allow it to fall below the level of the thermostat. When the new thermostat is in put the coolant back.

The temp gauge on the left of the panel is for the coolant. The temp reading in the trip computer is the outside temperature. It can fluctuate if the car is stationary for any time as the heat from the engine will affect the sensor.