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Everything you need to know about DPFs

Published 17 October 2017

Almost all modern diesel vehicles come with a Diesel Particulate Filter, or DPF. We explain everything from what it is and what it does, to recognising if there's a problem.

The Euro 5 emissions legislation, introduced in 2009, effectively made DPFs mandatory, so all diesel cars sold since then have one.

The name gives away what the DPF does for your car, it filters out toxic, microscopic particulates that are emitted through the exhaust. The aim is to eliminate 80 per cent of particle emissions.

What does a DPF do?

Diesel engines produce a lot of soot when they burn fuel. This soot is what's known as particulate matter. It’s a very fine substance that can cause serious health problems, namely breathing problems. 

The job of the DPF is to filter and store this soot in order to reduce emissions from diesel cars. But because they have a limited capacity, this soot has to be regularly burned off to regenerate the DPF. If the soot blocks the filter, this can stop the engine from running and leave you with a hefty repair bill.

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What is DPF regeneration?

Regeneration burns off the excess soot in the filter, reducing harmful exhaust emissions and helps to prevent the black plumes of smoke you sometimes see from older diesel vehicles when they accelerate.

There are two types of regeneration: active and passive. Passive regeneration takes place while driving, using the heat of the exhaust. Doing this on a regular basis is enough to burn off the soot and turn it to ash.

To make sure that the regeneration takes place, diesel vehicles should be driven for more than 15 minutes at more than 40mph. If the DPF can't passively regenerate, it will actively regenerate by raising the temperature of the exhaust gases by automatically injecting more fuel.

How will I know if regeneration is happening?

When the DPF is regenerating the cooling fans will run, fuel consumption will increase and there will be a pungent smell. If your car has start-stop, you may also find that it's deactivated during regeneration.

What happens if the DPF doesn't regenerate?

If this process is interrupted too often, the DPF amber warning light will appear. This usually happens to drivers that do lots of short trips, or lots of start/stop driving, because the vehicle doesn't get up to a high enough temperature to achieve passive regeneration.

Dpf Light

                           The DPF warning light usually looks like this.....

If this happens, you should immediately take the car out on a long stretch of road, like the motorway, for 15 minutes to give the DPF an chance to regenerate. If you ignore this warning light, the car will eventually go into limp mode to prevent further engine damage.

If left any longer, the DPF won’t be able to regenerate itself and will need to be cleaned or even replaced. Most manufacturer's warranties don't cover the cost of DPF replacement if the fault is deemed to have been caused by the owner's driving style.

You should also ensure you use the right engine oil. Certain oils contain additives that can actually block DPFs. Using low-quality fuel and running the car frequently on a low fuel level can also harm a DPF because the car may avoid regeneration in order to save fuel.

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What do I do if active and passive regeneration don't work?

You can buy DPF cleaner fluids, but it's unknown how well these actually work. Generally, it's a job best left to professionals, like Ceramex. If your garage doesn’t offer forced regeneration, you can ask if the DPF can be removed and sent to a specialist for cleaning.

What is the cost of a new diesel particulate filter?

If you don't maintain the DPF properly, excess soot can damaged the DPF beyond repair. If this is the case, it has to be replaced - usually for around £1000 or more. There are DPF suppliers that charge less, but make sure they have the correct Type Approval or they may not work correctly and end up costing you more in repairs.

Is it illegal to remove the diesel particulate filter?

Yes - it's illegal to drive a car that was designed to have a DPF without having one fitted. This is because should you remove it, your car will no longer meets its emissions standards. Owners face fines of up to £1000 for cars and £2500 for vans if caught. Removing a DPF can also invalidate your car insurance policy.

From February 2014, checking the presence of a DPF became part of the MoT test procedure. All cars that are designed to have a DPF now get inspected for one. If it’s missing it will mean an immediate failure.

>> Should I get a diesel car if I only do 8000 miles per year?

Some final advice

We recommend that if you only cover low mileages or lots of short journeys, you choose a petrol car instead of diesel. Which is why many small cars only come with petrol engines. Diesels don't make financial sense.

In fact, if you drive less than 15,000 miles a year, you'd be better off with a petrol car. Diesel cars are generally more expensive to buy new - it will take a long time to see a return on that extra outlay - even with the lower fuel consumption. Use our fuel calculator to find out exactly how long.

DPFs can last up to around 100,000 miles if maintained properly. After the car has exceeded that mileage, you could be looking at paying a large amount of money for a replacement - so always properly check MoT and service records when buying a used car. Otherwise, you could end up forking out for unexpected repairs on high mileage diesels.

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Comments

Telegram Sam    on 23 October 2017

Different economic criteria and considerations apply, I suppose, if like me you are driving a pre 2009 diesel?

GCW    on 23 October 2017

How can I find out if my car has a DPF? By number plate?

Pedro Mendes    on 23 October 2017

As usual, a well presented and explicit article in simple terms. Good old HJ, keep it up.

Colin Glaister    on 23 October 2017

It should be noted that 'fixed' cheating EA189 VW Diesels no longer passively regen, and now only active regen, thus making the car totally unsuitable for town driving. VW don't tell you when they applying the fix and deny that this will ultimately shorten the emission systems working life considerably. #VWFixFail

paul newonsmith    on 23 October 2017

Excellentwriteup, had I known this prior to buying my VW golf 2010 I would not have purchased it.
Since buying I have paid VW to replace the diesel particulate filterwhen the car went onto limp mode on the Motorway.
I will never buy another car from VW. Once bitten twice shy, theyve lost my vote. Excellent write up thank you.

DrTeeth    on 23 October 2017

Excellentwriteup, had I known this prior to buying my VW golf 2010 I would not have purchased it. Since buying I have paid VW to replace the diesel particulate filterwhen the car went onto limp mode on the Motorway. I will never buy another car from VW. Once bitten twice shy, theyve lost my vote. Excellent write up thank you.

You would have the same problem with ANY DPF.

Freeplay    on 23 October 2017

That article is out of date for many cars. From 2012 on VAG diesels don't passively regenerate they only do active regenerations, the exhaust gas never gets hot enough to burn the carbon, even at motorway speeds. The strategy for active regeneration is greatly improved and takes less than ten minutes and can be done at low speeds, no need to stay above 40mph. My 2.0 L TDi will regenerate at any speed, even at idle, few drivers are even aware the dpf is regenerating.

beufighter    on 23 October 2017

Makes you wonder, how many drivers Fail to read the owner's handbook. All deisel cars that i have owened and thats been a few since my first Peugot 305 in 1983, state in the hand book how to drive the diesel things. Run for a minute before setting off, run for a minute when you stop. Basicly take them for 'blast' once a week to get rid of the crap. Stop/start F...s deisel engines. Turn off stop start during long runs. When the motor stops,everything stops. Cooling, lubrication of bearings etc. Boiling oil carbonizes on the bearings etc etc.

Sory that should be 'the i before the e'

Edited by beufighter on 23/10/2017 at 16:19

Margaret Holmes    on 23 October 2017

Does my 2005 Merc C class 2ltr diesel car have a DPF?

NickNike    on 23 October 2017

Another reason why diesels should never have been encouraged for car use (except taxis). Well done Gordon, you idiot. And it makes me mad that my relatively clean, recent petrol car is taxed way above this diesel toxic rubbish. Yet again, politicians have no clue about anything.

BazzaB    on 24 October 2017

Since I retired and had to hand back my company car in 2003, I have used Honest John as my Car Care Advisor for all things car. I had never bought a car till I was 57! My first car in 2003 was a Skoda Octavia Est and next in 2013 a Skoda Octavia 2L Vrs (2007 reg) , both diesel. We have bought a Kia Rio petrol car this year for all general trips. Its very good for the money. The Vrs I bought at 103K, now 158K. It's never had a replacement DPF. I use this to tow a caravan and normally no short journeys. I constantly think the DPF is going to fail but it hasn't yet. In July, the DPF light came on after 200 miles whilst towing (after putting Shell's best diesel in as HJ advises) . I parked the caravan and drove up and down the M5 for 20 mins till it went out. Have just been to Cornwall (not towing and 900 miles there and back) and car was a dream!

Edited by BazzaB on 24/10/2017 at 11:41

Charles Harkin    on 24 October 2017

When the DPF warning light comes on the ECU puts the car into a different mode to aid the regeneration. Many people get very high mileages from their DPF, some well over 250K miles, ask some taxi drivers. Top quality oil and fuel make a big difference.

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