The most common cause is moisture trapped in the evaporator – the part of the air conditioning system that cools the air before it enters the cabin.
Every time you turn on the A/C, warm air comes into contact with the cold evaporator, creating condensation.
The water should drain through an outlet under the car, but if it gets blocked, moisture remains and creates ideal conditions for microorganisms to grow.
Spores, pollen, and organic debris accelerate this process, and an unpleasant odor eventually starts to enter the cabin.
A clogged cabin filter can also make the problem worse, as it reduces airflow and traps moisture.
If the ventilation smells bad as soon as you turn it on, the problem is almost certainly inside the system itself, not from outside air.
The first step is to replace the cabin air filter. If you haven’t done this in one or two years, it’s time to change it.
A dirty filter retains moisture and dirt, which promotes bacterial growth and is especially concerning for allergy sufferers.
If the bad smell continues, the evaporator needs to be cleaned. Mechanics use special foaming agents that kill mold and bacteria, applied through the vents or directly onto the evaporator.
A helpful trick is to turn off the A/C a few minutes before you stop driving and let the fan run – this helps dry the evaporator.
In more severe cases, a deep disinfection by a mechanic is necessary. There’s also ozone treatment – a gas released through the ventilation system that kills bacteria and unpleasant odors, instead of just masking them.
The best prevention is regularly replacing the cabin air filter and maintaining the ventilation ducts.
Use the “fresh air” option instead of continuous recirculation, as it reduces moisture buildup inside the cabin.
Before turning off the engine, let the fan run for a few minutes without the A/C to help dry the evaporator.
Also, parking in the shade helps because lower interior temperatures reduce condensation.
If you notice there's no water dripping under your car when the A/C is running, check the drain – a blocked drain will almost certainly lead to a bad smell.
With proper maintenance, you’ll prevent mold and bacteria growth, and your car’s A/C will smell just right – completely odorless!
