Here's why and what to do:
* Coolant Leak: The most likely cause is a leak in the heater core. The heater core is a small radiator located inside the car's dashboard that heats the air blown through the vents. If it cracks or develops a leak, coolant will mix with the air and be blown out the vents as white smoke (especially noticeable when the heater is on). Other potential, though less likely, coolant leaks might be from a cracked engine block or head gasket, but these would usually display other symptoms as well.
* Not Steam: While it *looks* like steam, it's actually coolant vapor. True steam would dissipate much more quickly. The persistence of the white smoke points to coolant.
* What to Do:
* Stop driving immediately: Continuing to drive with a coolant leak can lead to overheating, engine damage, and potentially a blown head gasket, resulting in very expensive repairs.
* Check the coolant level: Open the hood (carefully, the engine may be hot) and check the coolant reservoir. If the level is low, this strongly confirms a leak.
* Do NOT add coolant yet: Adding coolant without fixing the leak will just cause more to leak out.
* Have it towed to a mechanic: A proper diagnosis and repair are needed. A mechanic will be able to pinpoint the source of the leak (heater core, head gasket, etc.) and perform the necessary repairs. Replacing a heater core is a relatively involved job that requires dashboard disassembly.
Ignoring this problem will likely lead to a much more costly repair later. Get it checked out as soon as possible.
