Car Repairs - What Is A DPF And Why Do I Need It?

By: Motor Easy
Car Exhaust Fumes DPF Filters

 DPF/Diesel Particulate Filters - A Complete Guide

With more crack downs on the amount of exhast fumes entering our atmosphere, with the new 'T-Charge' coming into force for older vehicles driving into London, and all diesel and petrol cars expected to be abolished by 2040. More than 1,800 cars have failed their MOT test due to missing a Diesel Particulate Filter (DPF), new figures from the DVSA show, so why are people taking it out of their car and what can you do to check it is working and keep it working?


What does a DPF do?

When diesel cars burn fuel they produce soot, which can cause serious health and respiratory problems. The DPF is in a car to try and elimate up to 80% off this soot from leaving the exhaust and getting into the atmosphere. The DPF stores the soot in order to reduce emissions, but because they have a limited capacity this soot has to be burnt off often in order to keep it working properly. If your car is not able to burn off the soot this blocks the filter and leaves you with a hefty repair bill.


What does DPF regeneration do?

Regeneration is the process of burning off the excess soot in the filter, reducing harmful exhaust emissions, helping to prevent the black smoke you see coming out of older diesel engines when they accelerate. 

There are two types of regeneration; active and passive. Passive regeneration takes place while driving by using the heat that comes from the exhaust to burn the soot. If the car does this on a regular basis it is enough to burn off the soot and turn it to ash. 

In order to let passive regeneration happen your car needs to be driven at over 40mph for more than 15 minutes. If the DPF doesn't get this then it will increase the heat of the exhaust gases by automatically injecting more fuel, this is called active regeneration.


How will you know regeneration is happening?

You will know the process is taking place as the cooling fans will be running, fuel consumption will increase and there will be distinctive smell coming from your car. If your car has a stop-start function then this might also be disabled during regeneration.



Your DPF isn't regenerating, what now?

If regeneration is interrupted too often then the DPF amber light will come on your dashboard. This will usually happen when drivers do a lot of short trips, or lots or stop/start driving in traffic - as the vehicle doesn't get to a high enough temperature to achieve passive regeneration. 

If the warning light comes on it is recommended you take your car on a long stretch of road, like a motorway, for more than 15 minutes. If you ignore the light your car will eventually go into limp mode in order to try and prevent further engine damage.

If it is left any longer then the DPF will not be able to regenerate itself and will need to be either cleaned or in worst case replaced. 

It is also advised that you make sure you are using the right engine oil as certain oils can contain additives that block the DPF filter. Filling up with low quality fuel and frequently running your car at a low fuel level can also damage your DPF as the car may avoid regenerating in order to save fuel. 


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

If you don't maintain your DPF properly, excess soot can damage it beyond repair. If this is the case then the DPF will have to be replaced and can often cost £1,000 or more. There are some suppliers that will charge you less than this but you must make sure they have the correct Type Approval or this could cost you more in the long run as they will not work correctly. 


Can you just remove your DPF?

In short, no. It is illegal to drive a diesel car without a DPF, as by removing it your car will no longer meet emission standards. You could face a fine of up to £1,000 for cars, the cost of the new DPF, or £2,500 for vans if you are caught without one. DPF removal can also invalidate your car insurance policy.

From 2014 checking the presence of a DPF became part of the MOT test, with 1,800 cars failing due to this since then, figures from the DVSA have reported. Numbers are potentially much higher though, as the visual inspection currently carried out has been flagged recently as 'not fit purpose'. 


Diesel Particulate Filter Recommendations

If you are only covering short distances my advice would be that you buy a petrol car instead of diesel, in fact this is the reason many smaller cars only come with petrol engines. If you drive less than 15,000 miles a year it is recommended that you buy a petrol car. 

DPFs are designed, if looked after correctly, to last for up to 100,000 miles. After your car has exceeded that you are probably heading for a replacement so always check MOT hostory and records when buying a used diesel car to avoid spending through the nose for any unexpected repairs on high mileage cars. 


Further reading

Motornomics: How To Drive Your Car Forever

Car Repairs - What Would Frankenstein's Car Look Like?

The World's First Plugin Warranty

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