Why would the check coolant light come on in a 1999 Pontiac Montana?

Hey Leonard, Either it is low on coolant or the sensor needs to be cleaned. GoodluckJoe Also it could be a leaking intake gasket. To check for this look at your oil cap and see if it is white and also checkjust below the valve covers to see if there is any coolent and or oil on the sides of the engine the light will come on when, rad. fluid is low due to a leak, )many causes.) due to a bad hose ,loose clamp,a leaking water pump, or you are loosing fluid into your engine. check you oil and look for rad coolant fluid in it. you may have a blown head gasket. i just went through the same thing with the only exception being when i did notice the temp gauge went up past the normal running range i immediatly pulled over waited for the motor to cool down. When the teardown was completed we found not only was the intake leaking but i had a blown head gasket as well, best of luck to you. With the closed pressurized systems, the coolant can boil off if the gasket on the radiator cap gets old and hard. Replacing the cap is inexpensive. If it doesn't solve the problem then further investigation is needed.