The heater core is a small radiator that warms the car’s cabin using hot coolant from the engine. The cooling system’s hoses circulate liquid coolant through the heater core as they do through the engine and large cooling radiator behind the vehicle’s grille. A failed heater core is a common source of liquid in the vehicle’s front foot wells, with steam coming from the air vents or excessive fogging of inside windows. A sweet coolant odor is another sign of failure. Replacement costs depend primarily on labor: how easily the core can be removed, whether from the interior or the engine compartment.