Heater after running does not produce hot air. Blower works fine on 2001 Buick LeSabre

When heat is on the blower works well, but after warming up, the air coming out is luke warm or cold. Engine temperature around 180 degrees. Seems as engine gets warmer, air coming out of heater gets cooler

Asked by for the 2001 Buick LeSabre
Check coolant level, needs to be full.