If your vehicle is showing white vapor or steam blowing out of the a/c vents, here are the most likely causes and the parts you'll need to fix each one.
In humid conditions, cold A/C air condenses moisture into a visible mist. If the A/C drain tube is clogged, water pools in the HVAC box, making this misting effect much more frequent and often causing a musty odor.
A leak in the heater core allows hot engine coolant to escape into the ventilation system as steam. This vapor typically has a sweet smell and will leave a greasy, foggy film on the inside of the windshield.
A slow refrigerant leak causes the evaporator coil pressure to drop, making it freeze over with ice. As the ice melts or cold air blows past it, it creates a white mist out of the vents.
If the expansion valve is stuck open or malfunctioning, it feeds improper amounts of refrigerant into the evaporator. This causes the core to freeze into a block of ice, producing vapor as warm air hits it.
A faulty relay or temperature switch can cause the A/C compressor to run continuously without cycling off. This overcools the system, freezing the evaporator and blowing icy mist into the cabin.