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What is a diesel particulate filter?

The diesel particulate filter (DPF) is a key component of every modern diesel engine, but do you understand what it does or how to maintain it?

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The diesel particulate filter (DPF) is a key component of every modern diesel engine, but many people still don’t understand what it does or how to maintain it.

So if you’re questioning why your car does or doesn’t have one, or you want to know whether or not you should have yours removed, this simple guide will tell you exactly what’s what.

What exactly is a diesel particulate filter?

The DPF is, as the name suggests, a filter. It is fitted somewhere near the top of your car’s exhaust system, as close to the engine as possible to help heat the system up quickly when the car is started from cold.

Cutaway image of a diesel particulate filter from Vauxhall
Cutaway image of a diesel particulate filter by Vauxhall

The filter traps the toxic black soot produced by the engine in a bid to reduce the tell-tale cloud of black smoke that is produced by all diesel engines – usually visible when you put your foot down.

When the filter reaches a certain temperature, it burns up any soot particles in the filter and releases them down the exhaust pipe as much smaller particles. This is called ‘regeneration’.

So the particles still exist, but they’re too small to actually see and are far more comprehensively burnt up, which is supposed to reduce the carcinogenic and toxic nature of the particles (although this is heavily disputed).

Does my car need a DPF?

If your car was never fitted with a diesel particulate filter when it was built, then no, you don’t need to fit one.

However, if your car came with one, it’s a legal requirement and part of the MOT test. Cars built after 2009 (when emissions rules known as Euro 5 were introduced) will certainly have one, and some pre-2009 vehicles may also have one.

If you’re not sure whether your car has a diesel particulate filter, your local dealership should be able to tell you.

It is illegal to remove or tamper with the DPF or any of the car’s emissions systems. If you are found to have removed your DPF, you could be fined up to £1,000 (or £2,500 if you drive a van). It can also invalidate your insurance.

Under the new MOT rules introduced in May 2018, if the DPF is found to have been removed or tampered with, it’s considered a major defect and an automatic fail. The testing station should refuse to test the vehicle unless you can provide convincing evidence that you had a valid reason to tamper with the DPF.

If your car has a DPF, it is also an automatic fail if there is any visible smoke coming from the exhaust pipe.

In the six months after the new MOT rules were introduced in May 2018, the number of diesel cars that failed on emissions grounds has quadrupled from 58,000 to nearly 240,000.

Can’t the DPF get clogged up?

Yes it can, just like any filter. A DPF can become blocked with soot over time, which is usually caused by the filter operating below its most efficient temperature – usually at low speeds around town.

When a diesel engine is cold, such as when you start it up in the morning, it doesn’t burn the fuel very efficiently and produces a lot more of that toxic black soot. The gets trapped by the DPF, but that also needs to be hot enough to burn up the soot.

If you only take short trips, especially if you mainly drive around town and spend a lot of time idling at traffic lights, the filter doesn’t heat up enough to burn off the trapped soot, so it keeps accumulating.

When this happens, you will eventually see an orange warning light on the dashboard that looks something like this (or fairly similar):

Diesel particulate filter (DPF) orange warning light
A first-stage DPF warning light – time to take your car for a long drive

This means that your diesel particulate filter is nearly full, and you need to take action to clear it out.

How do I fix it?

Most of the time, a DPF can be cleared by being allowed to regenerate. This happens naturally when the car is running at speed and the exhaust is at its hottest. The heat in the filter causes the soot to burn away as described above.

Some cars have systems will also initiate a process to try and force the filter to regenerate. This is called ‘active regeneration’ and is caused by the car’s on-board computer pumping extra fuel into the engine. Burning this extra fuel raises the temperature of the exhaust, which should burn off the build-up of soot.

Taking a car for a spirited half-hour drive on a motorway will usually clear the fault, as the engine is working hard enough to get the necessary temperature into the DPF system.

What if I can’t or don’t do that?

If you ignore this orange light, things can get serious very quickly. The filter will completely block up and then you’ll see a red warning light, usually accompanied by a message to either go straight to the dealership or even to switch off the car and call for roadside assistance.

Range Rover diesel particulate filter (DPF) fail
If you get a warning like this, your DPF is blocked and needs professional intervention

If the filter is completely blocked, exhaust gases can’t get out from the engine to the outside world, and it could cause serious engine damage.

You can’t keep driving the car, so it has to go back to the dealership for the DPF to be removed and manually cleaned out. This will probably cost a couple of hundred pounds and is not covered by your car’s warranty.

On top of that, the filter can usually only be manually cleaned a couple of times before it becomes too damaged and needs to be replaced. This is very expensive (usually more than £1,000).

What can I do to stop the DPF clogging up?

Preventing problems with your DPF starts when you choose whether or not to buy a diesel-powered car. Car manufacturers and dealers have been trying to push every customer into a diesel car for the last decade or more, but that’s because it suits them – not necessarily you.

Short urban journeys don’t allow the DPF to get up to temperature, so you are far more likely to see the orange DPF warning light making a regular appearance in local driving. That means taking the car out for a half-hour blast just to clear out the filter – hardly an environmentally-friendly approach.

There are also many reports and forum discussions around the web (including here in our own forum) of certain vehicles with inherent DPF problems resulting from poor design and/or engineering by the car manufacturer – the Land Rover Discovery Sport and Range Rover Evoque are particularly notorious for DPF faults, which many owners have claimed make the cars virtually undriveable.

If you spend most of your driving time on short trips and urban driving, with few long trips in your normal routine, you should probably opt for a petrol car – or if you want an eco-friendly option, look at a hybrid or fully-electric vehicle.

If you do want to buy a diesel car, you should ensure the car gets a long run once in a while to allow the filter to regenerate. Ideally, this should be weekly or at least fortnightly to minimise your chances of your DPF clogging up.

Following the correct servicing schedule can also help, as can using the correct type of oil – some oils contain additives that may block the filter. You can also buy fuel additives that claim to help with DPF cleaning, although we’re not aware of any independent research that backs up these claims.

If you maintain the car correctly and your driving circumstances are suitable, there’s no reason why a diesel particulate filter shouldn’t last for 100,000 miles.

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Stuart Masson
Stuart Massonhttps://www.thecarexpert.co.uk/
Stuart is the Editorial Director of our suite of sites: The Car Expert, The Van Expert and The Truck Expert. Originally from Australia, Stuart has had a passion for cars and the automotive industry for over thirty years. He spent a decade in automotive retail, and now works tirelessly to help car buyers by providing independent and impartial advice.