Before starting, I know that the simple answer is to replace the sensor, but I'm curious by nature.
A couple of weeks ago, my coolant level indicator light came on for the first time. I checked everything out and the radiator and resevoir were full, so I figured it was the sensor and I'd replace it whenever. Well, I haven't gotten around to it yet, but noticed a curious pattern. The light only comes on when the coolant temperature gets close to the thermostat's opening temperature.
I would think that if the sensor was bad, it would be "bad" at all operating temperatures. I've checked levels several times in the past weeks and it doesn't appear to have lost a drop of coolant. So why does the coolant level light only come on when the engine's warm? And how does it even know that it's warm if it's just a "dumb" level sensor?

 

There's no rhyme or reason why they act that way. Just unplug it and be done with it.