DPF Regeneration
Diesel particulate filters (DPFs) became virtually mandatory from 2009 when the introduction of EU exhaust emission legislation known as Euro 5 meant diesels vehicles could not attain the standards required without having one fitted.
A DPF is a special ceramic filter encased within an exhaust box, which normally sits closer to the engine side of the vehicles overall exhaust system.
How does a DPF work?
The PDF allows gases and smaller, less harmful particles to pass, while capturing the larger soot particles within the filter.
Clearly, the DPF would not be able to capture exhaust soot forever, so periodically the vehicle should initiate an automated process known as regenerated to prevent the soot particles from clogging the DPF.
Although there are slightly varying ways of achieving DPF regeneration, in essence the large soot particles trapped inside need burning to produce much smaller ash particles and other, more inert chemicals.
How does DPF regeneration happen?
For the DPF to undergo a regeneration cycle, the DPF temperature needs to raise significantly, sometimes as high as around 600ðC. This can happen either passively or actively, but in either case, certain conditions must to achieved for the process to begin.
Passive DPF regeneration occurs when the DPF reaches the requires temperature to begin burning off the soot as part of the regular usage of the vehicle. This will occur typically when taking motorway or free flowing A-road journeys, as the DPF will only reach the required temperature if the vehicle is driven at around 40mph or higher for 20 minutes or more.
What do you do when you see the DPF light?
If a vehicle does not undergo sufficient passive DPF regeneration cycles to keep the filter free flowing, the filter will begin to block. The vehicle monitors the blockage level by reading the pressure drop through the filter from pressure sensors located before and after the DPF.
If this pressure difference reaches a pre-determined level, the DPF light on your dashboard will illuminate, indicating that an active regeneration is required.
Active regeneration is an automated process controlled by the vehicle, which raises the DPF temperature to the required level in a much shorter timeframe by injecting additional fuel into the system. As such, an active regeneration will typically take between 5 and 10 minutes to complete.
Despite an active regeneration cycle taking much less time to initiate and complete than a passive cycle, it still requires the vehicle to be driving constantly to work, so if the journey if too short, or interrupted, the process will fail to complete, leaving a partial blockage still in-place.
What do if your DPF light stays on, or turns red?
If your DPF light stays on, keeps coming back on repeatedly, or turns red, it's a clear signs that too much soot has built up withing the DPF and you will need to have the DPF unblocked by a professional.
Depending on the vehicle and severity of the blockage, the vehicle may also initiate a self-restricted mode, known as limp mode, where the vehicle can only drive at very low speeds, low engine revs.
There are numerous way to fix a blocked DPF, the most obvious being to replace it, at a cost of between £1,000 and £3,000, but the cheapest option to try first, is always a forced DPF regeneration.
While this is not always successful in fully unblocking the DPF, it usually removes enough of the built-up soot to allow the vehicle to take over managing the automated regenerate cycles again.
A DPF regeneration can only take place a certain number of times, before they become ineffective as begin to block with the ash, produced from the regeneration, rather than the soot.
In these instances, the DPF, will need to be removed and manually cleaned, or replaced entirely. However, given these are significantly more costly options, a forced regeneration, is always the most sensible options to try first.