Since 2009, diesel particulate filters have been present in the exhaust to reduce pollution and stop soot passing into the atmosphere.
The goal is to cut particle emissions by 80%, but the new technology is not without problems, with many breakdown situations being caused by cars with a blocked DPF.
Diesel particle filters need to be emptied on a regular basis to maintain performance, usually via a process known as ‘regeneration’, when the temperature in the exhaust is high enough to allow it – usually on faster roads.
The collected soot is burnt off, leaving very small tiny ash residue. Although the ash can’t be removed, if used correctly, a DPF in a car should be good for more than 100,000 miles.
A lot of cars don't get experience the correct right sort of use for passive regeneration to take place, so manufacturers have built in ‘active’ regeneration systems. When the engine control software senses the filter is becoming blocked, it injects additional fuel into the engine to build the exhaust temperature, therefore triggering regeneration.
Active regeneration is initiated around every 300 miles, depending on how a car is used – it takes between 5 and 10 minutes to complete. However, this can present a problem if the journey is too short and the regeneration doesn’t have time to finish.
Signs to look out for during active regeneration
• Auto Stop/Start does not work
• Fuel consumption is increased
• Cooling fans may be running
• A hot, sharp smell coming from the exhaust
• Faster engine at idle speed
• The engine may sound different
• Warning light indicating filter is blocked
In the case that you see a warning light to indicate that the filter is blocked, try to complete the active regeneration cycle and clear the light by driving for another 10 minutes, keeping to speeds over 40mph.
Do not ignore a DPF warning light – if you drive at slow speeds, or follow a stop/start pattern, soot will build up in the filter until the vehicle goes into ‘restricted performance mode’ to stop damage.
What to do if your vehicle goes into restricted performance mode
Driving at speed alone won’t be enough if your vehicle has entered into restricted performance mode; a dealer will need to do a manual or ‘forced’ filter regeneration. Sometimes, in extreme cases, it may be necessary replace the filter which can be costly – usually in the region of least £1000 plus labour.
In most cases, there’s only a short space of time between the DPF being partly blocked and it getting so blocked it needs a forced regeneration, so it is important to be vigilant.
It is recommended you drive mainly for around town, or ‘stop/start’, it would be best to avoid buying a new diesel car that is fitted with a particulate filter. Even if your driving isn't mainly urban or stop/start, you still may need to reassess your driving style to keep the system working properly and pay attention to the advice in your vehicle handbook.
Factors that prevent normal regeneration
Lots of short journeys where the engine doesn’t get hot and stopping and starting at regular intervals, including using the incorrect type of engine oil, could prevent normal regeneration from occurring.
Other factors include:
• A problem with the Exhaust Gas Recirculation (EGR) or fuel system
• A dashboard warning light or a diagnostic trouble code stored in the engine management system
• Exceeding recommended service intervals
• A low level of Eolys™ additive in the tank can prevent regeneration (if the vehicle uses it)
• Low levels of fuel, usually under a quarter of a tank
DPF fuel additives
Most DPFs are fitted near to the engine where the exhaust is hottest so that passive regeneration is more likely to take place.
Some cars use a different type of DPF which needs a fuel additive (Eolys™ fluid) to lower the ignition temperature of the soot particles so that regeneration can happen at a lower temperature.
Additive is stored in a separate tank, automatically mixing with the fuel, and a full tank of should last around 70k miles. It costs around £200 in fluid and labour for the additive tank to be refilled.
Once again, it is important not to ignore a warning light indicating that the additive tanks need refilling as you run a high risk that the DPF will quickly become blocked.
It is not legal to remove diesel particulate filters
It is not a legal option to get the DPF taken out of the exhaust, reprogramming the engine management software to avoid paying up to be repaired.
As DPFs are fitted to meet with European emissions regulations, it is an offence (under the Road vehicles (Construction and Use) Regulations) to use a vehicle that has been modified in this manner that makes it no longer compliant with the emissions standards it was designed to meet.
In addition, removing a DPF could also invalidate any vehicle insurance because it makes the vehicle illegal for on-road use. Since February 2014, if the DPF is missing where there was one originally fitted at stage of build, the vehicle will also fail its MOT.