Heater Blows Cold Air: A damaged or punctured heater core hampers heat transfer, leading to cold air blowing from your vent. Cabin Fogging Up After Warming Up: A malfunctioning heater core keeps the coolant cold. When warm air from the cabin meets this cold coolant, condensation forms on your windows, causing fogging. Unusual Sweet or Bitter Smell: Leaking heater core emits a distinct fruity or bitter coolant odor inside or outside the vehicle. This could also be a sign of a radiator leak. Dropping Coolant Level: A decrease in coolant level points to a potential leak in the heater core or radiator. It's a warning sign of overheating risk. Vehicle Overheating: A blocked heater core can cause your engine to overheat, leading to potential breakdowns on the road.