I have a 97 cadillac sts and I dont think the car is reading the coolant temperature sensor. When I let the antifreeze out of the car to put some new antifreeze in the car the check coolant level light didnt come on I can go about 20 miles then it tries to run hot I change the waterpump, and thermostat and the coolant temerature sensor.
Somewhat of a confusing post, you go from coolant level to an overheat. The level sensor is in the reservoir, it could be stuck indicating it is fine, or it is disconnected. If you find the connector and jumper the terminals, it should read low level. The level sensor is beside the point if you have an overheat. If it is overheating, and using coolant, I would recommend you go no further until you have the cooling system tested for the presence of combustion gases. It is not uncommon to have head bolt threads that have failed allowing a combustion leak past the head gaskets. There are alot of thrads on this site where people have exhausted substatial money in an attempt to solve an overheating problem without havin the block check performed. From my 15 years experience with Cadillac's, the most common cause for an overheating Northstar is an internal leak from failed head bolt threads.