All cars use various filters and they all require replacement at scheduled intervals, usually to coincide with other servicing. However, at times, they may need to be replaced earlier so here’s our guide to your car’s filters, what they do and tell-tale signs that they may need replacing.

1 – Air filter

Here we’re referring to the filtration of the air as it enters the engine’s inlet manifold (as opposed to the air entering the cabin, which is called a pollen filter or cabin filter – we’ll deal with that in a minute). It’s important that the air that is used in combustion is free from large contaminants, such as dust, leaves and insects. The air filter, fitted to petrol and diesel cars, takes care of that. Cars with two banks of cylinders, like V6’s or V8’s sometimes have two air filters, but vast majority have just one. They’re contained within an expanded part of the inlet ducting, often referred to as an ‘air box’ or the simply the ‘air filter housing’ and they’re usually very easy for the average person to replace. It is possible to extend the life of a given air filter element by blowing it through with compressed air, but the elements are not expensive so it’s hardly worth the effort, and it’s not always possible to visualise how effective the filter is being.

Do Your Filters Need Replacing?

Bear in mind that a dirty air filter is likely to reduce air flow to the engine, causing it to work harder to get the same volume of air. That will make itself known by increased fuel consumption and decreased performance. If it’s left a long time it may cause more obvious engine running problems, such as stuttering, misfiring or an inability to idle. A new air filter is usually close to white in appearance, while old ones are very obviously discoloured. Even if there are no symptoms, if you discover that the air filter is badly discoloured then it’s highly recommended that it is changed.

Here’s our video guide to replacing your air filter:

2 – Fuel filter

Fuel filters are generally quite different in petrol and diesel cars. In petrol cars they’re often a fit-for-life component and may not need to be changed at all. They’re also often located inside the fuel tank or in an ‘inline’ location on the fuel line underneath the car. Replacing the fuel filter on a diesel car is always part of the regular service schedule (probably due to the less refined nature of diesel it tends to have more contaminants in it) Diesel filters are usually located within the engine bay and are more accessible. It’s usually required to bleed the fuel system after replacing a diesel or petrol filter. In both cases they protect the fuel injection system from debris that may accumulate in the bottom of the fuel tank or fuel lines. Some diesel filters separate out water (as well as debris) that may be in the fuel, as water can lead to damage of expensive diesel pumps and fuel injectors.

Do Your Filters Need Replacing?

The most obvious symptom of a clogged fuel filter is lack of power under hard acceleration or up a hill. If the filter is partially blocked, it will restrict fuel flow, which, naturally, restricts the power output of the engine. A fuel filter that is left unchanged for a long time can lead to a build up of dirt in the injection system and could ultimately block an injector which would cause the engine to miss-fire.

We cover changing the diesel filter in our ‘How to service your car’ video

3 – Oil filter

Although air and fuel filters are important to smooth engine running, their ‘failure’ is nothing more worrying than reduced performance and fuel economy. Oil filters, however, have one of the most important jobs in the whole car, to filter the oil as it is pumped around the engine. Along with dust and dirt, tiny metal particles are circulated in the oil and it’s critical that the oil filter effectively removes these contaminants before they can reach important components such as the crankshaft and camshaft bearings. Hence, never, ever skimp on replacing the oil filter. It’s best to change the oil and filter according to the car manufacturer’s guidelines – at a minimum. Note that, if your car is used for mainly short journeys in and around an urban environment, you really should change the oil more frequently – along with the oil filter. Also, if you’ve decided to use long life oil, make sure that the oil filter is matched to it in terms of ‘life’, as that is not always the case.

Do Your Filters Need Replacing?

There are two basic designs of oil filter (one with an aluminium outer skin called a ‘spin-on’ filter and one without, called a ‘cartridge type’ – they’re not interchangeable), There are lots of different sizes and shapes so don’t assume one from car A will suit car B, even if it physically fits in. Make sure you compare the old and new, paying particular attention to the rubber seal sizes and positions. Don’t forget it’s vitally important to change the engine oil at the same time as the filter as all the contaminants are suspended within the engine oil

Here’s our video guide to changing your oil filter:

4 – Pollen filter

Pollen filters are a relatively new invention for use in vehicles, and while they have no effect on your car’s health, they could have on yours, so pay attention to suggested change intervals. As the name implies, these filters take pollen (and other contaminants) out of the air before it enters a car’s cabin. This helps alleviate hay fever for some, but a pollen filter also reduces fumes and dust from traffic and industry in built-up areas to the benefit of all car occupants. It’s often overlooked as a serviceable item, but it’s a good investment in your health.

Do Your Filters Need Replacing?

They usually last a few years and there’s no easy way to tell if they’re in need of changing. Really old ones will small bad, though. As they’re housed within the dashboard (and hidden away), they can be awkward to remove and replace, but don’t let that put you off doing it.

Here’s our video guide to replacing your pollen filter: