My 1997 buick century limited temp. guage kept going up. I added water and it stopped but now when im on the highway the low coolant light comes on. It goes off when i get off the highway. is that my thermostat?

is your electric fans running constantly. and most times then not you`ll need a water temp sensor or water temp relay. usually the temp sensor on that model comes usually next to the thermostat with a little wired plug on top. or on the radiator. either way pull the plug while the motor is cold. unscrew the sensor and screw the new one in and replace the electric plug back on top of the sensor. 2 things: Cold motor, have every thing set up and ready because when you take out the sensor the coolant will start to come out. you will not loose much if you work quick. keep rags or paper towels below the sensor to keep coolant off the motor and smelling when it gets hot. you have also pour or hose off coolant after your done installing the new coolant temp sensor.

intake gaskets or possible head gasket failure common on this engine

Sounds like the water pump needs to be replaced.

My low coolant light is on but it is not low

Have someone pressure test the cooling system - you likely have a head gasket leak

your buick century has a low coolant level sensor located in the passenger side of the radiator , sometiomes the sensor get covered with coolant sludge and will show that the cooling system is not filled , sometime the radiator is just still low on coolant . check the radiator when the engine is cold ( IF HOT WAIT !!! 30 mins ) and make sure the level is above the sensor.

only fill up the reservoir.

try adding it via the reservoir. make sure you have a 50/50 mix of water and antifreeze. If you want to cheat, cheat on the side of more antifreeze than water. Look for a leak in the hoses and water pump to find out if she has a larger problem that is causing the loss of coolant. look for drips under the car.

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..