The most common indications that an automotive heater core is failing include a foggy windshield, steam coming from vents when the heater is switched on, a water surging sound inside the dashboard, a sweet-smelling aroma coming from the vents and wet carpeting on the front passenger side. Heater cores are responsible for extracting heat from the coolant and transferring warm air into the passenger compartment.
Heater cores look like tiny radiators and heat passenger compartments by allowing hot coolant through small tubes with fins. Powerful fans then force air through these fins when heat is needed to warm passenger compartments.
There are several reasons why heater cores malfunction, but the common causes include continued temperature cycling from cold to hot which leads to leaky fittings, vibrations, rust build up and corrosion.
At times, low coolant levels and bad heater blowers are the culprits of insufficient heating. If warm air suddenly becomes cold while idling, inadequate coolant levels fail to reach heater cores that are normally mounted higher than an automobile's cooling system. This most often overlooked scenario could lead to clogging and eventual heater core failure.
According to Street Directory, flushing a clogged heater core sometimes improves efficiency, but replacing it is a better option.