Cars have two air filters. Everyone knows about the engine one, and almost nobody replaces the other, which is the one filtering the air you actually breathe for an hour a day. It is usually the most neglected consumable a household owns, and unlike most neglected things, its failure mode is unpleasant rather than invisible.

What the filter does

It sits between the outside air intake and the blower, catching dust, pollen, soot and leaf debris before they reach the vents. Better versions include activated carbon, which also reduces odours and some exhaust gases. As it loads, airflow drops and the trapped organic material sitting in a damp housing starts to smell.

Symptoms worth acting on

  • Weak airflow at high fan speed, often mistaken for a failing blower.
  • Musty smell in the first seconds after switching the fan on.
  • Slow demisting of the windscreen, which is a safety issue in winter.
  • Worse allergies in the car than outside it, which is the clearest tell.

The interval in practice

Twelve months or 15,000 kilometres is the common manufacturer figure. Shorten it if you drive on unsealed roads, sit in congested traffic daily, park under trees, or live somewhere with a serious pollen season. Lengthen it only if the car is barely used, and even then replace on time, because a filter degrades in place.

You will replace an engine air filter you never see and skip the one you breathe through. That is the wrong way round.

Doing it yourself

  1. Find the housing. Behind the glovebox in most cars, sometimes under the windscreen cowl or under the bonnet.
  2. Empty the glovebox and release the stops or the damper arm.
  3. Slide out the old filter and note the airflow arrow on its frame.
  4. Vacuum any leaves out of the housing.
  5. Fit the new filter with the arrow pointing the same way, and reassemble.

The whole job usually needs no tools. Freshy tracks car cabin filters at 12 months and shortens that to 8 on heavy use, which is the right setting for dusty roads or a long commute. Add one item per vehicle. The rest of the yearly jobs are in the complete schedule.

Questions

How often should you change it?

Every 12 months or 15,000 kilometres, every 8 months in dusty or high pollen conditions.

What are the symptoms?

Weak airflow, a musty smell, slow demisting and worse allergies inside the car.

Can I do it myself?

In most cars yes, in about ten minutes, usually behind the glovebox.