Your truck or car's engine takes in air as part of the combustion process, but that air needs to be free of particles such as dust and leaves. If not, the engine – and the cylinders and pistons in particular – could be damaged.
This is why an air filter is used. It's usually made from either paper or a mixture of paper and cotton, although aftermarket 'performance' filters, such as those made by K&N and AEM, use foam coated in a thin layer of oil to allow more air to pass through. Most filters are square or rectangular but some are round. The filter sits within a housing under the bonnet.
How often should an air filter be changed? With the exception of high-performance air filters, which tend to have a longer life, most air filters should be changed once every two or three years, or after around 36,000 miles. It's easy to forget, so make a note of when you change it so you remember next time.
Watch this video to see how to replace your air filter
Every Haynes manual shows you how to change your vehicle's air filter in easy-to-follow steps
Does your car or truck's air filter really need to be changed every two or three years? Can't you leave it for longer? As mentioned, a lot depends on how many miles you cover but also on where you drive.
For example, if you're in a rural part of the country where the roads are often dusty and there's lots of pollen, you may find that the air filter is pretty dirty after 12 months. If you an area where the air tends to be cleaner, the filter could still look as good as new after a couple of years.
Basically, it's best to use your common sense and not to let the filter get as filthy as the one shown at the top of the page.
We've saved the best part until last, because checking and changing your air filter is one of the easiest and cheapest maintenance jobs you can carry out on your car. In many cases it's easier to change than the cabin filter.
The latter is often tucked up behind the glovebox and filters the air before it comes through the cabin's air vents, removing pollen and even exhaust particulates.