If your vehicle is showing sweet smell coming through the interior ac or heater vents, here are the most likely causes and the parts you'll need to fix each one.
The heater core is a small radiator inside the dashboard that uses hot engine coolant to heat the cabin. If it develops a pinhole leak, sweet-smelling coolant vapor is blown directly out of the vents, often leaving a greasy film on the windshield.
The rubber hoses carrying coolant to the heater core can degrade and leak at the firewall connections. The sweet-smelling steam from the leak is drawn into the cabin's fresh air intake located at the base of the windshield.
If the cooling system was recently topped off or flushed, spilled coolant on the hot engine block or exhaust will burn off. This sweet-smelling vapor is easily pulled into the cabin through the cowl vents.
A leak from major cooling components can spray coolant onto hot engine parts. The resulting sweet steam gets sucked into the climate control system's exterior air intake while driving or idling.
While usually described as a chemical or musty smell, leaking A/C refrigerant and its associated lubricating oil can sometimes be perceived as sweet. This happens when the evaporator core inside the dash fails.