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What is a car's DPF (or diesel particulate filter)?

DPF

A DPF (or diesel particulate filter) traps harmful particles - or soot - emitted from a diesel vehicle’s engine and stops them from being released into the environment.

DPFs were first installed in the US in 1985 and they are now fitted to most diesel-engined wheeled vehicles - both on and off road - in the developed world.

A car’s DPF is fitted within the exhaust system, between the exhaust manifold and the catalytic converter. The filter traps the particulates and they are then dealt with by a process called regeneration, which occurs in one of two ways.

First, when the vehicle’s exhaust temperature reaches a certain temperature (on motorways, highways and other high-speed routes), the soot is burned to leave an ash, which is expelled via the exhaust. This is known as passive regeneration.

Sometimes if the vehicle is driven in town for extended periods, the DPF can’t regenerate. In this case, when it’s about half full, the ECU alters the amount of fuel burnt in the engine to raise the exhaust temperature.

You may notice higher engine revs when this is happening.

DPF

It’s when this active regeneration process is interrupted that problems begin. A DPF warning light may illuminate on the dashboard. If this happens don’t ignore it.

What do you do if the DPF light comes on?

You should take the car out for at least a 15-minute drive on a road where you can get up to more than 40mph. A motorway is ideal but not essential.

Try to keep the revs above 3000rpm for most of the journey by avoiding putting the car into top gear, or use the automatic gearlever to force the gearbox to select a lower gear.

Eventually the light will go out. Ignore the light and the soot levels will build until a point where the car will go into ‘limp home’ mode in an act of self-preservation, and you’ll need to take it to a dealer for forced regeneration or even a new DPF (which will be very expensive).

In the UK, it is illegal to drive a car that has had its DPF removed. While some unscrupulous garages offer this service, the vehicle will fail its annual MoT test when the emissions are measured.