I have to keep refilling radaitor tank every two days

most probably a failed/split transmission cooler in the radiator tank
If that is the case , the fix is a new radiator and a chemical flush of the radiator
get quotes from an accredited radiator shop after getting a proper diagnosis of the fault

CAR OVERHEATING FIRST CHECK FOR LOW COOLANT IN THE RADIATOR COOLANT OVERFLOW JUG.IF COOLANT LEVEL IN OVERFLOW JUG LOW REFILL ADD COOLANT BACK TO THE FULL COLD MARK.PUT RADAITOR CAP ON COOLANT OVERFLOW JUG START ENGINE LET IT IDLE UNTIL IT OPERATING TEMPERATURE.MAKE SURE TOP RADIATOR HOSE GETTING HOT WHILE ENGINE WARM UP.IF TOP RADIATOR HOSE DONT GET HOT WHILE ENGINE IDLING YOUR THERMOSTAT COULD BE STUCK CLOSE CAUSING CAR TO OVERHEAT.IF TOP HOSE IS GETTING HOT.CHECK FOR COOLANT LEAKS CHECK TOP RADIATOR HOSE AND BOTTOM RADIATOR HOSE FOR LEAKS.CHECK FOR LEAKS AT THE RADIATOR CORES AND PLASTIC SIDE CONTAINERS.IF ALL IS GOOD ENGINE OVERHEATING WHILE IN A LONG TRAFFIC LINE, COOLANT FANS NOT COMING ON.COULD HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR BLOWN COOLING FAN FUSE OR RELAY.COULD HAVE PCM FAULT.MAKE COOLING FAN ELECTRICAL CONNECTORS AND WIRING NOT DAMAGE OR BROKEN.

If the engine is running well and not running hot, chances are that the coolant level sensor has failed. You can either replace it or ignore the warning light and just check the level as "routine maintenance"
while checking your oil and other fluids weekly as every good vehicle owner does and has done since there were cars to drive.

You need to replace the thermostat. It is not letting the water go through it. They will burp out the tank when this happens.

first verify if the fluid is low or not, in the coolant tank and the radiator. if its low, refill it with water and have it pressure tested asap. if the radiator is not low, the sensor in the radiator is bad and will need replacement.

it goes in the coolant resorvoir, it is usually a clear plastic tank, that has full and low indicators on the side. fill to full mark when cold,

To clear the oil change light,turn the key to on,and press the accelerator pedal three times real slow within 5 seconds,this will clear the light,now on the coolant light sensor,low coolant,remove the sensor on the passengers side of the radiator,it has a wire clip,so use a small pick,with a slight hook on it,the sensor is square,and flat,You will see,it needs to be cleaned out,most likely with air pressure,then put the clip back on the sensor,and it will snap back on,refill the radiator.If this was at all helpful,please rate,I have a 2000 montana myself,

Is it the LOW coolant light? Your car is equipped with a sensor on the expansion tank (radiator overflow). Right front fender. It's there for when the car heats up. As the engine heats up and the coolant expands it racks into the reservoir/overflow/expansion tank. When the engine cools, it sucks the coolant back in. If your car was low on coolant then there's a good possibility that as it cooled, it sucked the recently added fluid in. Check that it is full to the "cold full" level by looking at the tank before you start it for the day. If it's not, then the sensor is telling you to add coolant. If it is, then the sensor in the tank may be bad.

change sensor in water tank

It is very likely you have a stuck float inside the coolant resevoir tank. This is very, very common. First verify that you are indeed low on coolant. If not, remove the tank, fill half way with warm water and drip in a few drops of dish soap. With the cap on and covering both openings, shake it like you're a mad man. Rinse it out then shake it and listen for a rattling noise. If you dont hear anything, time to replace the tank. The float is integrated into the tank.