My low coolant light comes on when I first crank the car.I can turn the car off after a few minutes and back on and the light goes off.Coolant level is fine.Is it a sensor?

 

probably the sensor. the sensor is in the cap. i had that problem in the 5.0 i had a while back. just get a new cap and all will be well.

 

Your floater is stuck most likely not the sensor. Check if the little bastard is floating if it is, then push it down and see if it comes up. Whats i did... Disconected the sensor, now perfect, no light That light is stupid and for people that never look under the hood.

 

I have a 99 GT with the 4.6, so not sure if yours is similar, but here goes. I drained and flushed my cooling system several times, then refilled with a premixed 50/50 coolant/water ratio. My coolant light would come on after a short period of time. I waited for the car to cool, removed the radiator cap, and looked at the float - which was sitting on the bottom. I moved the float up and down with a clean, long screwdriver, and confirmed it wasn't sticking. After thinking about it for a few minutes, I realized that during the flush, some water would still remain in the block, and my 50/50 mix was not bringing the entire cooling system up to the required 50/50 level. That float works on specific gravity I suppose, and will only float with the proper mixture. I drained out a gallon and a half of the coolant/water mix, and refilled with straight coolant. Voila! Float floated to the top of the tank, and light is off. For an added bonus, I unplugged the sensor, sprayed electrical cleaner on the contacts on both sides, and scrubbed with a toothbrush - it was a little gummy.
Hope this helps. I've read other posts where people thought the coolant/water mixture did not effect the float. This is not true - the mix definitely affects the float!