Why does my car heater smell sweet only when warm, could it be a coolant leak?

[Hannah30]

I’ve been noticing a faint, sweet smell coming from my car’s vents whenever I run the heater. It’s not there with the A/C, only when it’s set to warm. I’m worried it might be a sign of a coolant leak somewhere in the heating system, but I haven’t seen any drips on the driveway or a drop in the overflow tank level.

[IsabellaA]

I had this exact thing years back. a faint sweet smell when the heater was on, and I freaked out thinking coolant was leaking. I checked the expansion tank and the hoses under the hood; no drips, no puddles, and the reservoir stayed steady. In my case it was a marginal leak through a heater core seal that never left the driveway as a puddle, just a little vapor and odor. I ended up paying for a pressure test to be sure, but I didn’t actually see a big leak afterward.

[OliverG]

Could be cabin mold or a dirty cabin air filter. The sweet smell is common when the blower runs on heat and humidity. I swapped the cabin filter and ran it with fresh air on, helped a bit, but the smell didn’t vanish completely. Noted the smell only when heating, not when on A/C.

[Sofia_W]

I keep thinking about it, but maybe it’s not a mechanical issue at all. Sometimes the heater box collects moisture and something in the dash smells sweet when it dries. But I did notice the smell would show up even when the car sat in the sun for a minute, which makes me doubt an ongoing leak. Still not sure.

[Gary.T]

Have you checked if the smell comes from the vents consistently, or only after long drives?