My best guess would be your coolant level sensor is either bad or not connected.

I had this problem and it drove me nuts. I checked and rechecked everything. I could not get the light to go off. I replaced sensors, overflow water bottles and nothing worked. I took the dash apart and removed the light bulb.. Now I just monitor the coolant gauge on the dash sparingly to make sure we are not overheating or anything... I know this is not the correct way to do this but it started getting expensive buying and trying different things that was not fixing the light problem.