The four filters in your car, and what happens if you skip the wrong one

There are four, they cost little and they get mixed up. But neglecting one has very different consequences from neglecting another. The right replacement intervals for your car are in the manufacturer's service schedule.

Engine air filter

It holds back dust and insects before the air enters the cylinders. An engine draws in thousands of litres of air for every litre of fuel: the dirt that passes through there is enormous.

When it is clogged, the engine struggles to breathe, consumption rises and response becomes sluggish.

When: when the service schedule says, and sooner on dirt roads or in dusty areas.

Cabin filter (pollen filter)

It is the only one that serves the people rather than the engine. It filters dust, pollen and, if it is an activated carbon filter, part of the outside smells and gases too.

When it is saturated the sign is clear: the windows mist up and are slow to clear, because the air flow is reduced. Then comes the musty smell when you switch on the air conditioning.

When: when the service schedule says, often once a year; ideally before summer. On many cars it sits behind the passenger glovebox and it is one of the easiest parts to change yourself.

Fuel filter

It holds back impurities and water before the pump and the injectors. It is the most delicate of the four, and on a diesel it is the one that must never be put off.

In modern diesel systems the injection works at very high pressures, and the pump is lubricated by the diesel itself: if abrasive particles get through it wears from the inside, and the debris reaches the injectors. This is the fault that turns a cheap part into a very expensive repair.

When: at the interval given by the manufacturer. On diesels with a water separator bowl, the water that settles at the bottom must also be drained.

If a diesel hesitates under acceleration and recovers after a few seconds, the fuel filter is one of the first things to have checked, before the injectors, which cost much more.

Oil filter

It holds back the particles suspended in the oil. It is changed always together with the oil, never skipped: when it is saturated a bypass valve opens and sends unfiltered oil to the engine.

When: at every oil change, without exception.

In order of urgency

1. Oil: together with the oil, always.
2. Fuel: on a diesel, when due; putting it off is the most expensive choice on the list.
3. Engine air: when due, sooner if the roads are dusty.
4. Cabin: when due; it does not damage the car, but it concerns the air you breathe.

This advice is general: for the values and procedures for your car, refer to the manufacturer's handbook.