What are symptoms of a bad heater core on a 2000 ford expedition?

What are symptoms of a bad heater core on a 2000 ford expedition?

These may help:

http://www.ford-trucks.com/forums/1020140-heater-issues-2000-expedition.html

http://www.cargurus.com/Cars/Discussion-t628_ds495903

No heat or it leaks.

The first symptom is it's a 2000.