White smoke or steam coming from the heater vents is almost always coolant evaporating off a leaking heater core. The smoke is sweet, the windshield films up, and the carpet gets wet. Ranked causes below.
Pinhole or seam leak in the heater core. Coolant drips onto the hot core surface, flashes to steam, and exits the vents as sweet-smelling smoke. Definitive symptom.
A heater hose connection just behind the dash drips on hot surfaces and creates steam. Often easier to fix than a heater core, sometimes accessible from under the hood.
Some cars have a control valve in the heater hose circuit. When it leaks, coolant drips into the HVAC box and steams out the vents.
On humid days, switching from AC to heat can release a brief puff of condensation that looks like white smoke. No sweet smell, no wet carpet, no coolant loss.
Leaves and debris in the cowl can hold moisture that vaporizes when the heater warms up. Clear the cowl drains.
A blown radiator cap or overheating event pushes coolant up into the cowl, where it enters the cabin air intake. Look for coolant on the wiper cowl after an overheating event.
| Likely Cause | Typical Cost | DIY Difficulty | Severity | Likelihood |
|---|---|---|---|---|
| Leaking Heater Core | $400-$1,200 (labor heavy) | Pro Only | Medium | 75% |
| Heater Hose Leaking at Firewall | $30-$100 + 1-2 hrs | Moderate | Low | 35% |
| Heater Control Valve Leaking | $30-$150 + 0.5-2 hrs | Moderate | Low | 25% |
| AC Evaporator Condensation (Not Smoke) | $0 | Easy | Low | 20% |
| Clogged Cabin Air Intake | $0-$25 | Easy | Low | 15% |
| Engine Coolant Overflow Into Cowl | $0-$25 | Easy | Low | 10% |
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🔬 Get a full repair report →Coolant evaporating, almost always a leaking heater core. The smoke is sweet-smelling steam (ethylene glycol flashing off a hot surface).
Mildly toxic in high concentrations. In the brief amounts you would inhale in a normal drive, not acutely dangerous, but do not ignore it. Crack a window and get it fixed.
Coolant leaves an oily film that does not wipe off easily. Use a degreaser-based glass cleaner. Indicator that you actually have coolant in the air, not just water vapor.
Short trips yes, but you will keep losing coolant and risk overheating. Carry a gallon of pre-mix and check the reservoir before every drive.
Part is cheap ($50-$200). Labor is huge - often 6-12 hours of dash removal. Total $800-$1,500.
Stop-leak products sometimes plug pinholes. Risky because it can clog the rest of the cooling system. Use only for temporary repair to get the car somewhere.
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