Understanding Your Car: The Diesel Particulate Filter

Wednesday 17 October, 2012

Award-winning Audi, VW, Seat, Skoda and Bentley specialist RAC London is here to help you understand your car better. This month: what is the Diesel Particulate Filter?

First things first, what is it? The diesel particulate filter (DPF) is part of the exhaust system in a Audi, VW, Seat and Skoda car.  Since 2008, to comply with the latest European regulations, most diesel engines have had DPFs fitted.

In simple terms, the DPF captures and stores the harmful soot that’s a by-product of diesel engine exhaust emissions produced when the engine is running.

For cars that regularly travel on motorways at constant high speed, ‘passive regeneration’ takes place automatically.  This process burns away the soot in the DPF and is initiated by the engine control unit without the driver needing to do anything about it.

However, in cars that drive mainly around town, this process doesn’t get the chance to happen routinely.  So when the DPF gets to about 40% blocked, the engine’s control unit will try ‘active regeneration’.  This is when it adjusts fuelling to allow the exhaust system to burn hotter to try and dispose of the soot by burning it away.

By now the DPF warning light should be appearing on the dashboard. At this point the best thing to do is go for a motorway drive, following the instructions in the owner’s handbook. And if the process is successful, the warning light will go out.

But if this warning is ignored or the diesel particulate filter is not allowed to complete its burn cycle, it may become up to 75% blocked. More warning lights appear and there will be a lack of power from the engine.

Unfortunately by now there’s nothing a driver can do except come and see us so we can carry out a ‘forced regeneration’ of the DPF using our Audi, VW, Seat, Skoda and Bentley specific diagnosis machine. 

However, if the driver has ignored the final warning signs, it’s possible the DPF will be blocked to the point where even a forced regeneration won’t work. Then they could be looking at buying a new DPF at a cost of  over £1000.

The diesel particulate filter is an effective and eco-friendly engine feature. Look after it and it’ll keep you green for years to come.


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Fleet Manager, Fleet Van Servicing Centre North London.

Whilst travelling to you you ordered the part, and an hour after my initial call my van was on the lift and a new starter-motor was being fitted. An hour later I was on my way back to work. I cannot thank you enough for your help. Not only did you go out of your way to keep us working, you did it at a very sensible price.


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