Diesel particulate filter (DPF) - everything you need to know

If you own a diesel vehicle, then odds are you've heard about the Diesel Particulate Filter (DPF). Since 2009, all diesel engines require one of these filters to ensure that they are compliant with the Euro 5 standards for emissions. But you might still be wondering, what are DPF filters, why are they needed, and how do you maintain these essential pieces of kit?

What is a DPF on a car and what does a DPF do?

A DPF is a clever little device that's fitted to your diesel's exhaust system to capture harmful particulate; these are usually soot particles, giving DPFs the nickname 'soot traps'.

If you've seen an old-fashioned diesel in action, you probably noticed the puff of black smoke that it periodically emits out the exhaust, especially when it accelerates. The DPF prevents this - with modern filters able to capture up to 95% of harmful particles - and makes the whole system far more sustainable and better for the environment.

How does a diesel particulate filter work?

Before explaining why they are so useful, it's important to understand how DPF filters work. And as with most necessary things, it is ingenious in its simplicity.

First, the diesel will go through the diesel oxidation catalyst (DOC), which helps to convert carbon monoxide into the less harmful (to life) cardon dioxide. This is then pushed through the DPF, a series of honeycomb tubes that are blocked at the end, allowing the gas to escape while trapping particulates like soot (mostly soot truth be told).

How to avoid DPF problems

However, as with all filters, the DPF can eventually clog up, creating a blockage and hindering your vehicle's performance. To prevent this, a DPF filter must go through something called 'regeneration'. This is where the DPF burns off the soot that it has caught in a safe and environmentally friendly way, emptying the filter and making it good as new.

Most DPFs are able to do this automatically depending on how you drive your vehicle. If you do lots of motorway driving, your DPF will be getting hot enough naturally to regenerate regularly, keeping it clear and operating efficiently. This is known as 'passive regeneration'.

However, not everyone will be using their vehicle for long journeys, which is why modern DPFs are also able to do something called 'active regeneration'. This is where the engine actively adds additional fuel to raise the temperature of the exhaust when the filter reaches a certain level, usually around 45% full, and thus safely burn off the soot.

However, even with active regeneration, there may not be enough time to rid the DPF of all its soot, meaning that there will be an eventual build-up and blockage. If this happens, you may need to take your vehicle to a garage to go through 'forced regeneration', which is where a garage will artificially heat your exhaust to the point it burns off the excess soot.

How do you know if your DPF is blocked?

Thankfully, there is a very easy and simple way to know whether your DPF is clogged up, and that's through your car's warning lights. When the DPF is getting full, a light will illuminate on your dashboard, letting you know that there is some soot that needs burning off.

When the DPF is roughly 45% full of soot, the DPF warning light should illuminate on your vehicle's dashboard. This is to let you know that your vehicle will start 'active regeneration', adding additional fuel to burn off the soot caught in the DPF. After a few longer trips, the soot should be burnt off and the light will turn off again.

So, if you see this light, it doesn't mean that you need to immediately take your car to the garage. If the light is yellow, it's just letting you know the filter is a bit full and the car will start active regeneration soon, so you shouldn't worry if you hear your engine working a bit harder than normal or if your fuel consumption increases.

However, if the warning light stays on even after you've taken a few longer trips or if the light turns red, then that means there is something wrong with the DPF and you will need to take it to a garage to be fixed.

When does a DPF need replacing?

DPFs, like all car components, do suffer from wear and tear after prolonged use. The recommended time to replace your DPF is around 100,000 miles, so it's a fairly resistant piece of tech. Keep in mind, though, this isn't talking about blockages - a blocked DPF is a fairly common and easy problem to fix (unless the blockage is symptomatic of a larger problem).

When did a DPF become mandatory?

The first regulations introduced to help combat emissions from vehicles were launched all the way back in 1970, which really highlights how long we as a population have been tackling harmful emissions from vehicles.

However, the first Europe-wide regulations were only introduced in 1992 and focussed predominantly on ensuring that catalytic converters were mandatory for all vehicles across the EU - this was known as the Euro 1 standard.

Since then, there have been a growing number of Euro standards added, helping to protect air quality and limit the number of harmful emissions vehicles produce. As of this article being written, we are up to Euro 6, which was introduced in 2015.

However, it was Euro 5 that made diesel particulate filters (which were actually invented by Peugeot in 2000) mandatory on all new diesel vehicles. Euro 5 imposed strict limits on both the amount of particulates (soot) and nitrogen oxide emitted from diesel engines.

Both of these emissions have been proven to have negative long-term health implications and limiting them was one of the steps taken to try and protect the air quality in large cities.

Is it illegal to remove a DPF?

So why do some people want to do it?

Well, the DPF can inhibit your vehicle's exhaust system very slightly, so some believe by removing it, the performance of the car will improve.

Also, as active regeneration does add additional fuel to raise the exhaust's temperature, some believe that removing the DPF also improves the vehicle's fuel economy.

So, choosing between very minor improvements to your car's performance (if any) or the risk of being fined £1,000 or more? Sounds like a pretty easy choice to us!

Your questions answered

What is the purpose of a diesel particulate filter?

A DPF filters out particulates from your diesel car's exhaust, helping to prevent harmful car emissions from building up in highly populated areas.

Car emissions are considered dangerous for two reasons: they are greenhouse gases, so they affect climate change, and they have a direct negative impact on life. Nitrogen oxide is tied to an increase in respiratory and cardiovascular mortality, especially among children, and soot is confirmed as a carcinogenic.

While relatively harmless in small amounts, in areas of high concentrations of pollution, such as major cities, these emissions have a dramatic effect on the life expectancy of the population.

Where is the DPF located?

The DPF is a part of your vehicle's exhaust system, normally found just after the diesel oxidation catalyst and close to the engine to help naturally raise its temperature.

Do petrol cars have DPFs?

Of course, it's not just diesels that produce harmful emissions; if it was, there wouldn't be such a strong drive to switch to AFVs. And while petrol engines generally emit less particulates, some drivers may still want to limit their emissions, either for personal reasons or to help make their vehicle viable for Clean Air Zones, LEZs and ULEZs. This is where an on-board particulate filter (OPF) comes in.

While diesels are required by law to have a DPF due to the large amount of soot they produce, there is a petrol OPF equivalent. An OPF functions almost exactly the same as a DPF.

The main difference is that petrol engines don't produce as much soot and naturally burn at a much higher temperature, so an OPF doesn't fill up as quickly and is more likely to burn off any soot that is produced. This makes them much more reliable and are much less likely to block or need replacing.

How often does DPF regeneration occur?

Passive DPF regeneration occurs whenever your DPF gets hot enough to burn off the soot that has been caught in the filter. Active regeneration occurs when the filter reaches a certain threshold, usually around 45% capacity.

How often this occurs relies entirely on how much you drive, and how long your journeys normally take. The longer the journey, the hotter your DPF will get regularly and the more soot is burned off passively.

What is the fine for DPF delete?

The fine for tampering with the DPF is upwards of £1,000, making it a very expensive part of your car to mess around with.