The heater core is essential to your heater’s function of producing warmth. It works to allow antifreeze to go through the heating unit. Without it functioning properly, the heater won’t be able to give adequate warmth.
To check if the heater core is intact, follow these steps:
- Check for signs of antifreeze leaking., if you do this means it isn’t passing through the core.
- Look for abnormalities at the vent or defroster ducts. (eg. puffs of smoke, fog on windshield)
- Inspect the radiator and thermostat. If it works but there is no heat then the damage is likely to be at the core
- Open the hood while the car is warming up. If one hose is cold while the other is warm, the heater core is likely shot
- Flush and refill the cooling system. When it still doesn’t work that means the core needs replacing.
Source: How to Tell If Your Heater Core is Going Bad by Chris Moore of eHow