coolant sensor staying on

95 Lumina 3.1
Would like to know if the sensor is actually detecting mixture of water and antifreeze or is is just checking level of liquid in the system? Obviously by this question the car was filled with water and not with a 59-50 mixture of water and coolant.

Coolant is more electrically conductive than just plain water, so it is possible that water only would cause a low coolant light to come on. Plain water is bad, as it has no corrosion inhibitors for the aluminum components and has a lower boiling point, higher freezing point than coolant/water mixture, so get some coolant in her.

Had some air in the system and had that all taken out and found out that the radiator cap seal had a crack in her. So got the air out, filled her back up (put some coolant in it this time) and changed out the radiator cap. Thought I would be good. Not the sucker is losing the coolant, not sure where it is going and I get a fog inside the car from the vents. Can smell the coolant also. This engine had just been rebuilt, so needless to say I am scared to death I may blow this engine back up. I drive it nice and slow and make sure it is full of fluid before I go anywhere. When I had the radiator system checked they could not find anything, but now it is quite obvious the coolant is going somewhere.

any ideas

Fog from the vents? Oh-oh.

Check to see if the carpet on the passenger side at the firewall is wet. Sounds like busted heater core.

just replaced the heater core. the car came with a plastic heater core and the guy put a fully guaranteed metal heater core in it.

See if it is peeing coolant from the A/C core drain.

will get under the car tomorrow and figure it out with the guy who installed the new core. He seems to think it will be something we can fix tomorrow.