low coolant light on 2002 Buick Century

I changed out the waterpump but now the "low coolant" light comes on at starting and will not go off til run awhile then shut off and restarted. Coolant level is right up in rad and overflow container.

Asked by for the 2002 Buick Century
Check the coolant level sensor, it may be sticking when cold, and check that the level is OK when the engine is cold.