[dcarrb]

The only way a heater core could malfunction would be if it were leaking, and even then you'd still have some heat. Or it could conceivably be clogged. More likely there's a problem with the mechanism(s) that control airflow through the core.

Did you try raising the temperature setting to see if you get heat at any point? One of mine starts out fine but the heat wanes the longer I drive. On trips of an hour+ I have to continually bump-up the thermostat to stay comfortable.

dcb