Smelly Car Air Con: Causes and Fixes

Where the smell comes from

Deep behind the dashboard sits the evaporator, a small radiator-like part that gets very cold when the air con is running. Moisture from the cabin air condenses on it, just as it does on a cold glass of water. That moisture is supposed to drip away through a drain tube under the car.

When the evaporator stays damp and dark, and dust and pollen settle on it, mould and bacteria can start to grow. Every time the fan blows air across that surface, it carries the smell into the car. The odour is typically worst when you first switch the system on, after the car has been standing.

• Musty or damp smell on start-up: often growth on the evaporator.
• Sour, vinegary smell: commonly bacteria building up in a damp system.
• Sweet smell with misting windows: can point to a coolant leak in the heater system.
• Burning smell: not an air-con odour, so stop and have it checked.

The cabin filter

The cabin or pollen filter cleans the air before it enters the system. Over time it fills with dust, leaves, pollen and moisture, and can become a source of smells itself. A damp, clogged filter can also reduce airflow, which makes the system work less effectively.

Changing it is one of the simplest and most worthwhile maintenance jobs, and often it is the first thing to try. The replacement interval depends on the car and where you drive, so check the handbook, and consider changing it sooner if you park under trees or drive through dusty areas.

Blocked drains and water where it should not be

If the evaporator drain tube becomes blocked by debris, the condensation cannot escape and the case behind the dash stays wet. In worse cases, water can back up and spill into the passenger footwell, leaving a damp carpet and a persistent musty smell even when the system is off.

A wet carpet that never seems to dry is a clue worth acting on. Other sources of damp, such as a leaking door seal or a blocked scuttle drain, can produce similar smells, so the cause needs to be tracked down rather than assumed.

What actually fixes it

The right fix depends on the cause. A new cabin filter deals with a spent filter. A cleared drain tube deals with standing water. For growth on the evaporator, an air-con cleaning treatment is used, in which a cleaning product is introduced into the system to break down the deposits and rinse away. Whether a particular treatment is suitable depends on the car and the way the system is built.

A service that includes air-con servicing and a check of the filter, drain and system condition usually gives a good chance of the smell not returning. Sprays that only mask the odour will not remove the mould, so the smell tends to come back.

Keeping it from coming back

A simple habit helps a great deal. A few minutes before you reach your destination, switch the A/C compressor off but leave the fan running. That lets the evaporator dry out, so there is less damp to feed growth. Using the air con regularly, even in cooler months, also keeps the system circulating.

Replace the cabin filter on schedule, avoid leaving the vents on recirculation for long stretches, and fix any water leaks promptly. At Banks of the Tyne in Hebburn we carry out air-con servicing and diagnostics, so if the smell persists we can look into what is behind it.

Frequently asked questions