A heater core is a small radiator-like device located inside your car's dashboard. Its job is to transfer heat from the engine's coolant into the passenger cabin to keep you warm. When you turn on the heat, a blend door opens, and a fan blows air over the hot heater core, warming the air before it circulates inside the car.
Think of it as a secondary, much smaller radiator. Your engine's cooling system circulates a mixture of antifreeze and water (coolant) through the engine block to absorb excess heat. This hot coolant then flows through a hose into the heater core. The core itself is made of narrow tubes and fins, which maximize surface area for heat exchange. The blower motor forces air from the cabin across these heated fins, and that air is what you feel coming out of the vents.
The most common sign of a failing heater core is a lack of heat. However, more telling symptoms include a sweet, syrupy smell inside the car (from leaking coolant vapor) and a film of mist on the inside of the windshield that's difficult to wipe away. This film can be oily because coolant contains antifreeze. A significant leak will also cause the coolant level to drop, potentially leading to engine overheating. Replacing a heater core is often a labor-intensive and expensive repair because the entire dashboard usually needs to be dismantled to access it.
| Symptom | Underlying Cause | Implication |
|---|
| No heat from vents | Clogged heater core or stuck thermostat | Ineffective cabin heating |
| Sweet smell in cabin | Coolant leaking from heater core | Early stage of failure |
| Foggy windshield (interior) | Coolant vapor entering HVAC air intake | Confirms an active leak |
| Coolant loss with no underhood leak | Internal leak into HVAC system | Requires professional diagnosis |
| High labor cost for replacement | Dashboard removal required for access | Major repair expense |
