2000 monte carlo, low collant light comes on when

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how are you checking the coolant level? if by opening the cap, that is the wrong way.
with the heater at full and the car well warmed up, check the coolant in the recovery containe, and fill to hot if needed. then carefully openn the bleeder air valve(there may be two on some models), until a whisp of coolant starts to come out then quickly close the valve. it looks like a bolt on top of a bolt. if it has a hex head its 7mm if it has a slot then use a screw drive(the slot head was later used and the hew discontinued).
DO NOT OVER-TIGHTEN THE HEAD, as it easily will break off!!!
when ALL the air is out the coolant is now full and the light should no longer light, if it continues the sensor mounted on the passenger side of the radiator has gone bad or is not plugged in.