How do they work?
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.
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.
Mainly town based driving
If your own car use is mainly town-based, stop/start driving it would be wise to choose petrol rather than risk the hassle of incomplete DPF regeneration.
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.
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.