Diesel particulate filters - everything you need to know about your DPF
A diesel particulate filter can cut emissions by 90% but also could cost you a packet to replace. We explain what it is and how you can look after it...
Despite their dwindling popularity among new car buyers, there are approximately 11 million diesel-engined cars on the UK’s roads. Many owners drive diesel cars because they typically return excellent fuel economy, are often very good tow cars and the way they deliver their power makes them easy and relaxing to drive.
However, in recent years, interest in diesel models has fallen – partly due to concerns about emissions. That’s where diesel particulate filters (DPFs) step in: required on all Euro 5-compliant diesel models registered in the UK since 2009, the DPF helps to reduce the car’s tailpipe emissions.
Nevertheless, if it becomes clogged, it could cause serious issues – not least a repair bill that could span from £750 to £3000. So, what is a DPF and why should you pay attention to it?
Here, we share how a DPF works, why they become clogged and whether the risk of an expensive repair bill should put you off buying a diesel car altogether.
How does a DPF work?
Compared to petrol-engined vehicles, diesel cars produce considerably more soot, which can significantly worsen air quality, particularly in urban environments. To prevent soot from being emitted into the atmosphere, a DPF is designed to collect the particles.
Whatever the car, the process works in much the same way: exhaust gasses pass through the filter, leaving large soot particles behind. As a result, the amount of particulate matter emissions produced by the engine can be reduced by up to 90%.
However, over time, soot particles can clog the DPF, preventing the exhaust gasses from passing through. This can result in a number of negative symptoms including reduced performance, increased fuel consumption, difficulty starting the engine and a strong smell of diesel inside the car.
How do I look after my car’s DPF?
To prevent the filter from being clogged, the soot needs to be burned at a high enough temperature to turn it into smaller ash particles, which can then be expelled from the system. It’s when this cleaning or ‘regeneration’ process is interrupted that the majority of issues with DPFs occur. There are two types of regeneration, which attempt to remove soot from the filter in different ways.
Passive regeneration
Passive regeneration normally takes place automatically at higher speeds, when the engine is running at higher revs.
To make sure that the regeneration takes place, most manufacturers suggest that every few hundred miles, the car is driven for a period of more than 15 minutes at a consistent speed of more than 40mph. Doing this should kick-start regeneration and clear the filter.
Active regeneration
If soot can’t be cleared from the DPF via passive regeneration, the car’s onboard computer will try to simulate a high-speed drive in a process called ‘active regeneration’.
During an active regeneration, the temperature of the exhaust gasses will rise for around 10 to 15 minutes. During this time it’s likely you’ll notice your car’s stop-start system will deactivate, if it has one.
How to clear your DPF, and what to do if you see a DFP warning light?
There are a number of reasons why either regeneration process is unable to take place – the primary cause is often due to the diesel car only being used for short journeys that don’t allow the engine to warm up to its optimum temperature.
If the filter is clogged and neither type of regeneration can be completed, an orange DPF warning light will appear on the instrument panel.
If this happens, the best thing that you can do is immediately take the car out on a dual carriageway or motorway for 15 minutes of continuous driving to give the DPF an opportunity to regenerate.
If you ignore the warning light and continue driving in stop-start traffic at a slow pace, expect its colour to change from orange to red and other warning lights to illuminate. Eventually, the car will also go into ‘limp home’ mode to prevent further damage to the engine.
If left for much longer, the DPF won’t be able to regenerate itself and will need to be cleaned or even replaced. Should the DPF need to be replaced, most manufacturers' warranties will not cover the cost if the fault is deemed to have been caused by the owner’s driving style and not by a fault with the filter itself, leaving you facing a potential four-figure bill.
As well as making sure your car has a high-speed run regularly, you should always ensure your fuel tank is at least a quarter full. That’s because many systems won’t begin the regeneration process unless it’s determined the car can complete the cycle – which requires more fuel – without leaving you stranded at the roadside. And as ever, it’s critical you use the correct engine oil.
Should you buy a car fitted with a DPF?
The potential of having to spend thousands replacing a clogged filter has caused some drivers to ask whether they should buy a diesel or petrol-engined car. This depends on the mileage you cover and the sort of driving you’ll be doing.
If the majority of your journeys are short or primarily take place at urban speeds, and longer trips that use dual carriageways and motorways are rare, you’re usually better off choosing a petrol car over a diesel model. While petrol-engined cars typically offer slightly worse fuel economy, you can prevent headaches and the risk of an expensive repair bill.
In addition, if you typically cover less than 12,000 miles, you may also be better off with a petrol. That’s because diesel models are typically more expensive to buy new, compared to petrol cars, and it’ll take a long time to make that money back in fuel bills.
Nevertheless, if you do a lot of faster driving and cover a considerable mileage each year, you should have no problems with a car with a DPF. Even so, you should still follow the recommendations in the car’s handbook and pay attention to any dashboard warning lights that may appear.
This also applies to drivers interested in buying a used car with a DPF; however, it’s worth remembering that the filter generally only lasts for 100,000 miles if it’s been cared for properly. As a result, if you’re interested in buying a used diesel-powered car with a high mileage, you may have to pay a large amount of money for a replacement filter.
Can’t the DPF just be removed?
Driving a diesel car that has had its DPF removed is illegal and could land you a fine of up to £1000. This is because, without the DPF fitted, the car will no longer meet its emissions standards.
Since February 2014, checking the presence of a DPF has become part of the MOT test procedure. All cars that are designed to have a DPF now get inspected for one, and its absence will mean an immediate failure. Removing the DPF from your car could also result in complications from your insurance provider.
If you’re considering buying a used diesel car that you suspect should have a DPF fitted, make sure you ask the owner if it has been removed and check that the car has a recent MOT test certificate.
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