Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
First confirm that the radiator is full of coolant and both heater hoses going into the heater get hot when the engine is warm.
Is the engine warm and thermostat open? Is there actually a thermostat in there? Did the heat ever work before?
How low is it? If it's only an inch or two, just use water, unless of course the coolant mixture is weak. Is it? Test it.