Buick 1999 century replaced the sensor light and still comes on.

Have you checked to see if a fuse has been blown that operate the fan? Check your fuse box and book for the info concerning your radiator fan or any fan operation. That's a good start.

In the radiator overflow bottle.

Replace sensor.

Your cooling sensor is in the white plastic coolant tank that is on your firwall under the hood. If you fill it with coolant and the light continues to stay on. try and slap it a few times and see if that resets it. it could be gunked up. if that doesnt work fill the tank with radiator flush and let it sit for a little while and then flush the tank and refill with coolant.

yes

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

u need to buy the special coolant to your car i have the same car whit the same problem and is cause to the incorrent cooland the cooland to your car is the  DEX-COOL  if u dont find that u need some hit       (silicate-free) 

Philly, the fuel filter is attached to the frame in front of the gas tank, under the driver side of the car. You must relieve the pressure in the fuel line before you start, and it is also recommended that you disconnect the neg. battery cable. After disconnecting the neg. cable, remove the gas cap. Next, look under the hood on the fuel rail for a shrader valve (like the type on a bicycle tire). Place a rag under the valve, remove the cap, and press the valve in with a plastic pen to relieve the pressure in the line. You can now go under the car to remove the filter. You can disconnect the fuel lines easily with an inexpensive plastic disk made for this purpose (only a few $ for several sizes at a parts store). Be careful of fuel in the line that will drain out, no smoking or sparks during this procedure. Should be 1 bolt securing filter to frame. Note arrow on new filter, and install in the correct direction. Replace cap on shrader valve, put gas cap back on, reconnect battery cable. The low coolant light is probably a faulty sensor signaling that it's low, when it's actually ok. Look for wires coming from the reservoir bottle, you should be able to find the sensor at the end of the wires to replace it. If you change the fuel filter in a garage, leave the door open for air circulation, gas fumes can explode easily, having the door open will help dissipate the fumes.

is there coolant in the system ? if there is go to the coolant bottle there is a sensor in the bottom of that bottle if there is any thing beside coolant in that bottle it will cause the sensor to act erraticly. if there is no coolant our low coolant then the sensor is working you need to look for a leak

The coolant level sensor is located on the radiator. If you are looking at the engine from the front of the car it will be on the left side. There will some kind of clip holding it in the radiator. Removed the clip and pull the sensor out. You will lose coolant so try to swap it out quick or drain some of the coolant out. Once new sensor is installed, install clip to keep the sensor in place and reconnect the wires to the sensor. Crank the engine and make sure the coolant level is full. This should correct your problem.