Tank may look full,but squeeze the radiator hose slowly with the tank cap off. First the fluid will go up so don't squeeze too hard. When you release the hose it will pull the fluid down in the tank. Do this a couple times and your tank will need to be topped off,once topped off you should not have the light any longer.Goes without saying ,do this when the engine is cold. If you open that cap with a warm engine,you'll get burned and fluid will shoot out all over your engine.

 

Also, the tank needs to be filled right up to the brim for it not to register low coolant. mine looked full and was giving the same warning.

 

And, the float tends to stick. It sounds sad, but gently but firmly smack the tank and free it up. Sort of like spanking a small child. You don't want to hurt it, but you want them to know you are there.