6.2L check coolent light

i have a 1991 van with the 6.2. recently the check coolent light came on. i checked the level, topped it up, it didn't need much, but since then the light is almost always on, occasionally after it has been running for some time the light will go off. sometimes it stays off till i shut it off and restart, other times it will just come on again after a while. Does anyone know what this problem could be.

sounds like the sensor is going.