Every smell has a place, and all three places are on one path
Most guides start with “change the cabin filter”, and about six times in ten that is right. But the sentence that actually matters comes before it: work out which of three places is producing the smell first, because one of them is not fixed by a filter at all, and one of them is not a smell problem — it is water in the wrong place.
The three places sit in a line behind the dashboard. Outside air comes in under the windscreen, passes through the cabin filter, crosses the evaporator — the cold aluminium fins that dry and cool it — goes through the blower, and arrives at the vents. The water the evaporator pulls out of the air has to leave through a drain tube to the road. Filter, fins, drain. That is the whole list.
The check that separates them costs nothing and takes ten seconds
Turn the fan to full and ask one question: is the airflow weaker than it used to be? A filter is a sheet of paper, and paper loaded with dust blocks air. That is geometry, not judgement — a clogged filter always costs airflow, and nothing else on the list does. Biofilm on the fins is a film a few microns thick on the surface; it stinks, and it does not measurably change how much air gets through. Water in the air box does not either. So if the airflow has dropped, you are almost certainly looking at the filter, and you know it before you have opened anything.
The second check is timing. Is the first breath the worst? The evaporator runs at 0–5 °C and is wet the entire time the air conditioning is on. When you switch off, that water stays on the fins — dark, warm, dusty, and covered in everything the air brought in. Overnight, whatever is living there multiplies and produces acid. In the morning the first gust of air carries a night's worth of it in one go, and after that it is diluted and fades. A filter cannot produce that curve: a filter smells the same from the first second to the last. Worst in the first minute and fading is the fingerprint of a wet evaporator.
The third is fog. Does the windscreen fog up in a way you cannot clear, and does the passenger carpet feel damp underneath? The top of that carpet is waterproofed, so it feels dry; the foam below it holds litres. Wet foam in a closed box is a mould factory, and a permanent source of water vapour inside the car — which is why the glass never quite clears. That is the drain.
The one habit that costs nothing, and why nobody does it
Two minutes before you arrive, switch the A/C off and leave the fan running. Condensation stops, the fan blows room-temperature air across the fins, and they are dry when you park. Nothing sits wet overnight, so nothing grows. That is the entire prevention, it is free, and it takes about two weeks of doing it before the smell fades noticeably — which is exactly why most people give up on it and buy a can of something instead.
Air freshener and a charcoal bag are not repairs, they are covers. Fragrance adds molecules and removes none, and scent layered over mildew is a harder smell to live with than the mildew was. A charcoal bag works by passive adsorption, which needs air moving through it; in a parked car the air barely moves, and a bag that has been in a hot car since spring is saturated and starts releasing what it took. Both are worth buying — after the source has been dealt with, not instead of.
Where it stops being a DIY job
Cleaning the evaporator looks like a ten-minute job on video and is not one. The usual method — foam sprayed into the outside-air intake — sends foam through the blower and its resistor first, and a soaked resistor is a dead resistor, on plenty of cars behind a dashboard that has to come out. The fins are also behind the blower, so the foam gets to the part you can already reach and not the part that smells. A shop uses a camera and a low-pressure injector through the drain, or a fogging machine, and reaches the fin faces you cannot see.
The drain tube has the same trap in a cheaper form: clearing it with full workshop air pressure splits the tube or blows it off its spigot, and then the condensate goes into the footwell by design. Low pressure, or a flexible wire, gently. And if the carpet is genuinely soaked rather than damp, stop and think about what is under it — on many cars the body control module and a fistful of connectors live in that footwell, and a wet module is an electrical fault, not a smell.