A diesel particulate filter (or DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.

They are designed to burn off the accumulated particulate either passively through the use of a catalyst or by active means such as a fuel burner which heats the filter to soot combustion temperatures; engine programming to run when the filter is full in a manner that elevates exhaust temperature or produces high amounts of NOx to oxidize the accumulated ash, or through other methods. This is known as "filter regeneration". Cleaning is also required as part of periodic maintenance, and it must be done carefully to avoid damaging the filter. Failure of fuel injectors or turbochargers resulting in contamination of the filter with raw diesel or engine oil can also necessitate cleaning. The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF. If the driver ignores the warning light and waits too long to operate the vehicle above 40 miles per hour (64 km/h), the DPF may not regenerate properly, and continued operation past that point may spoil the DPF completely so it must be replaced.

Regeneration

Passive regeneration

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

Active regeneration

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 is a bit stop/start the regeneration may not complete and the warning light will come on to show that the filter is partially blocked. Source

Expensive Repairs

If you continue to ignore warnings and soot loading keeps increasing then the most likely outcome will be that you will have to get a new DPF costing around £1500.

 

BBC WATCHDOG ON DPFs - Watchdog has received over two hundred complaints about problems concerning diesel particulate filters. And it's a problem that affects diesel cars made by most manufacturers. Some drivers have had to pay hundreds of pounds to have their filters cleared whilst one customer needed a replacement costing over £1000.

 

Jaguar S Type XJ diesel particulate filter recall - The DPF on 2.7 Litre V6 diesel engine vehicles may exhibit smoke and/or fire from the underside of the vehicle, flames emanating from within the rear exhaust tailpipe or an orange glow from the underside of the vehicle. Heat from a fire within the DPF can radiate to the surrounding vehicle components and may lead to heating of the underside of the transmission tunnel and subsequent melting of the interior components and potential interior fire.