It may be fuelling high as the coolant sensor is telling the ECU that the engine is 'cold' and as a result it is pumping a richer mixture through. I would try changing the sensor first, whilst you are at it, check your tyre pressures and also check the state of the airfilter (infact, check when it was last serviced and when it is next due for servicing).
EDIT: Coolant sign on the dash also comes up when the expansion tank sender decides that it wants to play up - a new one is about a tenner and should sort it out, oh and its nice and white, and not a light brown colour as they tend to discolour over time so it's easier to make a visual check of how much coolant you have in there.