Most cars have two air filters, and they do very different jobs. One protects the engine. The other protects you. Both are cheap compared with the problems they prevent, and both are easy to overlook. Here is what each filter does, when to change them, and why fit matters as much as the filter material.
The engine air filter
An engine burns a huge amount of air. Every litre of fuel needs thousands of litres of air, and all of it passes through the engine air filter first. The filter traps dust, sand, insects and other debris that would otherwise wear the cylinders, damage the turbocharger or contaminate the mass airflow sensor.
A blocked engine air filter restricts airflow. On modern engines the control unit will usually compensate, but you may notice:
- Reduced performance, especially when accelerating
- Higher fuel consumption
- A dirty or contaminated airflow sensor over time
The cabin air filter
The cabin filter, sometimes called a pollen filter, cleans the air that comes through the heater and air-conditioning vents. It traps pollen, dust, soot and road grime before it reaches you and your passengers. Many cabin filters also include a layer of activated carbon that helps reduce traffic fumes and smells.
Signs the cabin filter needs replacing include:
- Weaker airflow from the vents, even on full fan
- Windows that take longer to demist
- A musty or damp smell when the fan or air conditioning is switched on
- More sneezing for hay fever sufferers
How often should they be changed?
Service intervals vary by manufacturer, engine and how the car is used. As a rough guide, many cabin filters are changed every 12 months or at each annual service, because they clog with pollen and moisture whatever the mileage. Engine air filters often last longer, commonly two to four years or a set mileage. Your service schedule is the reliable source for your car.
Change them more often if you:
- Drive on dusty or unmade roads
- Spend a lot of time in heavy city traffic
- Suffer from allergies or asthma
- Notice any of the warning signs above
Where to find them
The engine air filter sits inside a plastic air box in the engine bay, usually held shut by clips or screws. The cabin filter is often behind the glovebox, under the dashboard, or below the scuttle panel at the base of the windscreen. The manufacturer's parts diagrams show the location and every clip and seal involved. Our guide to exploded parts diagrams explains how to read them.
Why genuine filters are worth it
A filter is only as good as its seal. If the edges don't sit perfectly in the housing, air takes the easy route around the filter instead of through it, and unfiltered air reaches the engine or the cabin. Genuine filters are made to the exact shape of the housing, with the correct seal profile and stiffness.
Genuine filters are also designed for the airflow the engine or ventilation system expects. A filter that is too dense can starve the engine. One that is too open lets fine particles through. Cheaper cabin filters may also leave out the activated carbon layer, even if they look similar from the outside.
Finding the right part number
Filters can change between engine sizes, model years and facelifts, and some cars have different cabin filters with and without air conditioning. Use the part number on your old filter or check it against your VIN. Our guides to using your VIN and reading VW Group part numbers can help.
A quick tip when fitting
Most filters have an arrow showing the direction of airflow. Fit it the right way round, clean out any leaves or debris from the housing, and make sure the cover clips fully home. It only takes a minute and makes a real difference to how well the new filter works.
Don't forget the rest of the service
Filters are usually changed alongside the oil, oil filter and, on some cars, the fuel filter. Replacing them together keeps the service history simple and makes sure nothing is missed. If you are doing the work yourself, note the date and mileage in the service book or keep the receipts, as a full history helps when you come to sell the car.
Shop genuine filters
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