Heater not blowing hot air

i would check or replace thermostat. if that dont work it could alsob be the heater core or coolant low. i have a 02 celica gts with the same problem i checked the thermostat it was fine its my heater core . but water started leaking the pass floor board . you ca check the thermostat by boiling water in a pan and put it in.. if its good it will start to open up if not its bad