although this is an old post I just want to let people know that on my car it seems the previous owner used some stop leak in there and it clogged up the sensor which was on the upper right side of my 92 lumina.
It just has a metal pin which comes off then the sensor is pulled out be careful because antifreeze will come out in the process.
then just remove the sensor and if it has anything clean it out I cleaned mine out with a toothpick.
then I reinstalled mine and filled it with antifreeze and No more light.
hope this helps someone.