A/C Evaporator

It sits in the heater box next to the heater core. The blower pushes air through it. Moisture from the air condenses on its fins and drains under the car, which is why a puddle of water under a parked car with A/C on is normal. A temperature sensor on the evaporator prevents it from freezing.

Liquid refrigerant enters at low pressure, evaporates and cools the fins; the air passing through is cooled and dried.

A musty smell often comes from bacteria on the damp evaporator; cleaning treatments and a new cabin filter help. A blocked drain can let water into the footwell.