Low coolant light, temp normal

intake gaskets or possible head gasket failure common on this engine

Go to a dealer service department and buy a new coolant level sensor and install it in your overflow tank. A parts store is unlikely to have one unless all GMs use the same part.

The sensor in the coolant tank has gone bad. It's the module mounted on the plastic overflow tank - with two wires running to it. You can purchase a new one at any auto parts store and install it in less than a minute, or you can just disconnect it.

Bill B

My low coolant light is on but it is not low

You may want to check your manual. There is usually a light to tell you the coolant level in your reserve or overflow tank is low, and there is another light to tell you the system is overheating due to not enough coolant in the entire system. Nonetheless if either light remains on and you have refilled the overflow/reserve tank it is time to look for a leak. Leaks can be at hoses, in gaskets, and other locations. A puddle on the floor where the car is usually parked can help you locate a cracked hose or loose clamp, an antifreeze smell in your exhaust can help you locate a much worse problem.Often you may detect an antifreeze smell only after the car is warmed up which can be a leak which is running onto an exhaust pipe.

Sounds like the intake gasket or even possibly the head gasket have gone south if everything else has been replaced. Internal engine leaks will cause engine to overheat very easily.

The thermostat must be failing. Consider replacing it.

I think your buick has a pressure coolant system so you may have air in the system and need too bleed it and for the light coming on while on the highway most likely low coolant when your driving the higher rpm the more coolant is drawn into the motor and triggering the light from coolant reservoir running low on coolant

only fill up the reservoir.

I have a 1997 Buick Century with the same problem. Both my fans work if the A/C is
on as they should. If your turn off the A/C the passanger side fan will not come on
and at an idle the coolant in the radiator will boil, you can hear it in the overflow tank.
If you are driving there is enough air flow thru the radiator that the coolant doesn't
boil and the car doesn't overheat. I have replace the water pump, the thermostat.
and the ETS or engine temperature sensor mounted just above the thermostat in
the thernostat housing, to no avail, I still have the same problem. The only other
thing is the coolant level sensor in the radiator, which I haven't replace yet due
to the fact you have to pull the radiator in order to replace it, which is a major
undertaking. I have a low coolant indicator on all the time on the dash so I have
a pretty good idea the coolanat level sensor is bad. The coolant level is not low
so that sensor is not working as it should. I hope this may help with your problem.
To test you fan raise the hood when it is cold and top off the radiator and fill the
overflow tank to the cold mark. Start the car and let it idle with the A/C off and
see if the drivers side fans comes on. It will have to get to operating temp
before the fan will come on. If it doesn't come on replace the radiator low coolant
sensor on the passanger side of the radiator. If that doesn't fix the problem
replace the thermostat and the Engine Temperature Sensor at the thermostat
housing with an A/C Delco Engine Temperature Sensor. Good Luck..