Do you not think that is completely mis-leading if they do not put a simple figure in the handbook? I want to use this as another point in my argument against them.

quote from the AA

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If you use a car mainly for town-based, stop/start driving it would be wise to choose petrol rather than a diesel with Diesel Particulate Filter (DPF) because of the possible hassle of incomplete 'DPF regeneration'.

The car exhaust emissions standards (Euro 5) for new diesels have effectively required fitment of a DPF since 2009 although many cars registered before 2009 will have had one fitted too in anticipation of the change in standards.

Standards aim to deliver an 80% reduction in diesel particulate (soot) emissions but the technology's not without problems – AA patrols are regularly called to cars with the particulate filter warning light illuminated indicating a partial blockage.

And even if your driving isn't mainly urban/stop-start, changes to driving style may be required to get maximum benefit from these systems.

Diesel Particulate filters (DPF) or 'traps' do just that, they catch bits of soot in the exhaust.

As with any filter (think of the bag in your vacuum cleaner) they have to be emptied regularly to maintain performance. For a DPF this process is called 'regeneration' – the accumulated soot is burnt off at high temperature to leave only a tiny ash residue. Regeneration may be either passive or active.

Passive regeneration takes place automatically on motorway-type runs when the exhaust temperature is high. Many cars don't get this sort of use though so manufacturers have to design-in 'active' regeneration where the engine management computer (ECU) takes control of the process.

<*** src="http://www.theaa.com/motoring_advice/Images/dpf-warning-light.jpg" alt="image of a dpf warning lignt" /> When the soot loading in the filter reaches a set limit (about 45%) the ECU can make small adjustments to the fuel injection timing to increase the exhaust temperature and initiate regeneration. If the journey's a bit stop/start the regeneration may not complete and the warning light will illuminate to show that the DPF is partially blocked.

It should be possible to start a complete regeneration and clear the warning light simply by driving for 10 minutes or so at speeds greater than 40mph.

If you ignore the light and keep driving in a relatively slow, stop/start pattern soot loading will continue to build up until around 75% when you can expect to see other dashboard warning lights illuminate too. At this point driving at speed alone will not be sufficient and the car will have to go to a dealer for regeneration.

If warnings are still ignored and soot loading continues to increase then the most likely outcome will be a new DPF costing around £1000.

The most common type of DPF features an integrated oxidising catalytic converter and is located very close to the engine where exhaust gases will still be relatively hot so that passive regeneration is possible.

There's not always space close to the engine though so some manufacturers use a different type of DPF which relies on a fuel additive to lower the ignition temperature of the soot particles so that the DPF can be located further from the engine.

The additive is stored in a separate tank and is automatically mixed with the fuel whenever you fill up. Tiny quantities are required though so a litre of additive should treat around 2800 litres of fuel, enough to cover 25,000 miles at 40mpg.

Systems using a liquid additive introduce another service cost to be taken into account too – we've heard reports of dealers charging up to £200 to re-fill a half empty tank.

With this type of DPF regeneration will be initiated by the ECU every 300 miles or so depending on vehicle use and will take 5 to 10 minutes to complete. You shouldn't notice anything other than perhaps a puff of white smoke from the exhaust when the process is completed.

We're seeing some evidence of these systems failing to regenerate too, even on cars used mainly on motorways. It seems that on cars with a very high sixth gear engine revs are too low to generate sufficient exhaust temperature. Occasional harder driving in lower gears should be sufficient to burn off the soot in such cases.

Unfortunately there's no evidence in AA breakdown data that the problem's going away – newer models seem just as likely to sufffer DPF problems if not driven 'correctly' as those built when DPF's were introduced.

If you buy a car with a DPF fitted it's important to read the relevant section of the vehicle handbook so that you understand exactly what actions to take if the warning light illuminates and how, if at all, your driving style may need to be adjusted to ensure maximum DPF efficiency and life.