Diesel Particulate Filters

Posted on November 29th, 2013 by GEM Motoring Assist

Few recent technological developments have caused as much angst among motorists, as diesel particulate filters have, but Rob Marshall advises that they are reliable, provided the car is driven appropriately.

Thirty decades of diesel engine development has seen them transform from being sluggish and smoky to peppy and polished. While DERV-powered cars have the attraction of being far thriftier to live with, consumers have demanded performance to rival (and even beat) petrol units. Even though diesel engines produce lower levels of certain pollutants, such as carbon monoxide, when compared to petrol alternatives, the trade-off has been increased emissions of soot, or ‘particulates’, and, unlike most tailpipe pollutants, they are visible to the naked eye. Yet, legislation has forced carmakers to adopt means of reducing soot levels, through not only more efficient injection technology but also the introduction of post-combustion treatment developments.

Diesel Oxidation Catalysts appeared on diesel exhaust systems from the mid-1990s approximately, which were joined by the first diesel particulate filters (DPF) at the beginning of this century. As the name implies, the filter traps soot and prevents it from entering the atmosphere. However, a filter cannot hold an infinite quantity of soot and a method has to be found of disposing of the waste, without releasing it into the atmosphere.

Diesel Particulate Filters

The bulky Diesel Particulate Filter is located beneath the car, bolted to the exhaust system, as close to the engine as possible

Heating the exhaust-mounted DPF is the solution. When the soot reaches approximately 600 Degrees Celsius, it vaporises to an ash, which is harmless and most of it passes the filter and exits through the tailpipe.

Diesel Particulate Filter

A cross-section within a typical Diesel Particulate Filter (courtesy of BMW)

Pressure sensors tend to be located either side of the Diesel Particulate Filter and, should the engine ECU detect the filter starting to block, it will heat up the exhaust itself. This occurs usually by an injection of diesel into the engine’s cylinders, after the combustion stoke, when the exhaust valves are open. The unburned fuel enters the DPF, where it is ignited and increases the internal temperature significantly. This is called Active Regeneration but the process tends to take place at speeds above 35mph, when the engine is at its optimum running temperature. In low-speed urban conditions, especially when the engine coolant is cold, the procedure is not activated.

If Active Regeneration fails, because the car is not driven in the appropriate conditions, the DPF will continue to clog, until the driver is alerted by a dashboard warning lamp. Should the warning be ignored, the DPF can become so blocked-up that the engine will revert to its emergency ‘Limp Home’ mode and workshop attention will be required. Sometimes, a garage might be able to force the DPF to regenerate on a ramp but total cleansing of the filter is not guaranteed and an expensive replacement might be the only solution.

Diesel Particulate Filter

The typical soot trap and regeneration cycle of a DPF. (courtesy of Vauxhall)

On some models, typically cars where the DPF is located further away from the engine, a chemical is dosed automatically into the diesel fuel, to reduce the regeneration temperature from 650 degrees Celsius to approximately 400-450 degrees. This chemical tends to be stored in a separate reservoir and requires topping up between 40,000 and 80,000 miles, depending on make and model. It can cost around £250.00.

Ignoring any low-additive level dashboard warnings is unwise, because the DPF will be unable to regenerate as efficiently, which will shorten its working life and increase the risk of the car entering its limp-home mode or even breaking down.


As the DPF is not intended to be worked upon by a DIY mechanic, owners can prolong its life and reduce the chance of a premature blockage by being aware of which conditions to avoid.

Try to avoid prolonged stop-start runs. Should you cover a lot of urban driving, try to drive on a clear road at a constant speed of at least 50mph (presuming that it is safe to do so), allowing the engine to rev under load at around 2,000rpm for approximately fifteen minutes. This should be sufficient to activate the regeneration cycle, should it be needed.

Yet, regeneration will be interrupted, if you stop the car. Should the engine’s ECU attempt to restart an Active Regeneration Cycle continuously, because the previous cycle had been incomplete, unburned diesel can wash down the engine cylinder bores and contaminate the engine oil. Apart from wasting diesel, in extreme cases, this can cause engine damage, resulting from the oil level being raised beyond the maximum level on the dipstick..

All engines burn oil but certain anti-wear additives, such as sulphated ash, phosphorus and sulphur (SAPS), build up within the DPF and cannot be removed, even by regeneration. Therefore, most manufacturers that equip their cars with Diesel Particulate Filters insist on low SAPS oil being used. Failure to refill or top-up an engine with the correct low SAPS oil will cause premature clogging of the DPF. More details about choosing engine oil is here.

Should you see a DPF-associated warning lamp illuminate, do not ignore it. Check your owners’ handbook and follow the advice given.


Some companies offer a service of unbolting the DPF, removing the innards, prior to replacing it and re-programming the engine ECU.

GEM Motoring Assist does not sanction this. Apart from permitting higher levels of diesel particulates to be released into the atmosphere, it is a modification that should be declared to your insurance company. In the future, it is possible that the presence of a standard-fit and fully-functioning DPF could become a mandatory MoT Test requirement.