Separate the source from the moment you notice it
A stale smell when the fan starts can make the whole climate system seem suspect. First note whether it appears only at the beginning, continues throughout the journey or occurs even with ventilation switched off. Also notice whether changing between outside air and recirculation changes it during ordinary use. These observations help separate air entering the cabin from moisture or material already inside.
Check simple cabin sources before assuming a hidden mechanical problem: a damp coat left in the boot, a wet removable mat or spilled liquid under a seat can remain noticeable for days. Remove the source and dry accessible items appropriately. Do not dismantle seat wiring, push tools into vents or spray household products into openings. A fragrance may hide the smell while leaving the moisture untouched.
The cabin filter and moisture in the ventilation system can be relevant, but an odour alone does not establish which part needs attention. Look at the maintenance record and the handbook's guidance for the correct filter. If the job is not an owner procedure on your vehicle, ask a workshop to inspect it. The location and access can differ considerably between models.
Ask for the cause to be considered
Tell the workshop if there is recurring mist on the windows or a damp footwell as well as the smell. A cleaning service may be appropriate in some cases, but a blocked drainage path or water ingress needs its own investigation. Ask what will be inspected before agreeing to a treatment described only as deodorising. The aim is to understand why the condition keeps returning.
Do not use ozone generators or concentrated disinfectants in an occupied vehicle, and do not mix cleaning chemicals. Follow product and vehicle instructions for any ordinary cleaning you undertake. If the odour is instead sharp, fuel-like, exhaust-like or associated with smoke, stop treating it as a routine musty-vent complaint and seek appropriate help. Describe the change clearly rather than relying on the word 'smell' alone.
After cleaning or repair, check whether the original start-up odour and any associated dampness have changed during normal use. Keep the findings if the problem returns. An air freshener that makes the cabin pleasant for a day does not demonstrate that an underlying moisture source has been removed.
Note whether the odour returns immediately or only after the car has stood overnight; those are different patterns to bring back to the workshop.
