2004 Porsche Cayenne • 50,500 miles

Every morning, it takes a longtime to get the car warmed up and get good warmed air even if I put heater on Hi
from time to time, the coolant temp gauge needle shoots straight down to 0 and the radiator cooling fan kicks on with high speed. Each time I have to stop the car on side of the road, use a hand held code reader to clear the code P3081 and coolant temp needle will back up, when I restart the car the radiator fan stops running.
everything becomes normal but the coolant temp only stays around 140 f degrees.
i guess that the thermostat is always in open position, thus failed to block the flow of coolant inside the engine when it is cold. Also when I drive on the highway with no traffic, the coolant temp needle will lower to 130 f
with the cold wind blows in.
can you help me to pin point the issue? Thanks
February 15, 2013.

I think you already know what the problem is, the thermostat. As far as the temp gauge, you may have a bad temp sending unit.