low coolant light on but water tank full

you might have air in the system and it won't take the coolant form tank. loosen the exspansion tank bleed screw then retighten to 1 nm remove cap on a cool engine add 50/50 coolant mix. set heat adn blower on high, run through inst panel vents. run car until warm air comes through heater. let cr cool remove exspansion tank cap afill tank once again with 50/50 mix. do this until full