If it is leaking it is always bad, and you will most of the time have a sweet smell. There is sometimes a film on your windshield.
possibly a leaking heater core look for antifreeze dripping into the interior of the vehicle. usually it is the core or hose connected to the heater core.
usually you will get antifreeze smell and steam inside coming from defrost
Check for a line leak to your heater core or a leak in the heater core it self. Usually it is under the dash on passinger side.
A bad smell coming from the heater in a vehicle can indicate a bad heater core. One way to check this out is to see if the smell is worse when you sit in the passenger seat. If it is, there is a good chance the heater core is bad.
Leaking heater core.Leaking heater core.
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If it's a sweet smell then its probably a heater core leaking antifreeze.
This is a classic tell-tale sign that the heater has a "blown" core. A new heater core is the only option, I think, and that runs to money because of the labor involved. If you live in a very warm climate, you can get by with simply bridging the "In" and "Out" heater hoses wit a connector made for the purpose. But if you need the heater at all in winter weather, the core must be replaced.
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Those are the classic symptoms for a heater matrix (core) leaking
Usually this is an indication that you have a leak... either in the heater core itself, or the hoses connecting to the core. This is also usually accompanied with a greasy film on the inside of the front window (which is coolant). I suggest this is a repair best done by the pros.