Whether or not you know much about it, if you own a diesel car that was built after 2009, then your car will have a DPF. This diesel particulate filter – or DPF – is located in your car’s exhaust, and it works to limit the harmful emissions your car produces.

In order for your DPF to run as it should, it goes through a process known as regeneration, be that active or passive. These two forms of regeneration help to get rid of the soot that builds up in your car’s exhaust – this soot is the by-product of the diesel your car burns off.

Without the regeneration process, this soot build-up could easily block your DPF, which could pose further issues for your engine, and may leave you in need of an expensive DPF replacement.

If you want to learn about DPFs in more detail before reading this article, you can check out What is a Diesel Particulate Filter? (And Why Should I Care) to gain a better understanding.

Now that you have a basic knowledge of what your car’s DPF is, let’s learn a little bit about the process of active and passive regeneration.


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What Is Active Regeneration?

Active regeneration is a process in which extra fuel is sent to the exhaust, so that its temperature can be raised in order to burn off the excess soot which is starting to block your DPF.  Active regeneration is particularly useful for those of us who tend not to drive long distances.

Your car will struggle to burn off the excess soot without active regeneration if this is the case, as it isn’t in use long enough to complete the regeneration cycle. Your car’s electronic control unit (ECU) will recognise when this process is needed and instigate it accordingly.


What Is Passive Regeneration?

Passive regeneration is another way in which your DPF is designed to empty itself. In order to trigger passive regeneration, you need to take your car for a longer drive, so that the exhaust can reach a higher temperature. You can do this by driving your car at a higher speed on the motorway, as the combination of the speed and the increase in temperature can create a chemical reaction in your DPF which can neutralise the soot.

A layer of ash will be left behind, which over time can harden and damage the DPF. Whilst soot can be removed with DPF cleaning, the DPF may need to be removed, cleaned, or replaced if too much ash has accumulated, as a regeneration will not get rid of it.

If you think that neither active nor passive regeneration is making a difference to your blocked filter, or you are worried about your car's worsening performance, then you can book your car in for a DPF clean to help burn off some of this soot. 


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Will I Know When DPF Regeneration Is Happening?

You will be able to tell that active regeneration is taking place if you notice any of the following:

  • Your engine sounds different to normal
  • There is a hot, unpleasant smell coming from the exhaust
  • Your cooling fan is running
  • Your engine idle speed is faster
  • Your car's automatic start/stop technology isn't working
  • Your car is using more fuel

These are all normal symptoms of active regeneration. As for passive regeneration, if you drive your car on the motorway for 15 minutes or more, at a speed of around 40mph, then you should trigger passive regeneration.


How Often Does DPF Regeneration Occur?

Active regeneration happens roughly every 300 miles and is triggered by your car’s electronic control unit (ECU) once your soot level is approaching 40 to 45%. Passive regeneration should occur when you take your car on longer journeys at higher speeds.


How Long Does DPF Regeneration Last?

Active regeneration can last 5 to 10 minutes, whilst you would need to drive for 15 minutes or more at a speed of 40mph or higher to trigger passive regeneration and allow the regeneration cycle to complete. Your DPF will not be able to complete the cycle if:

  • There isn't enough fuel in the fuel tank
  • The fuel light is on
  • The engine management light is on
  • The pressure sensors or pressure pipes aren't working
  • You aren't using the right engine oil, or the oil is contaminated
  • Your fuel additive level is low

Be aware of these factors, and you can help your car to properly complete the DPF regeneration cycle.


Can I Force DPF Regeneration?

When you find that neither active nor passive regeneration seem to be unblocking your DPF, then it is time to take your car in for professional DPF cleaning. The process of forced regeneration can take up to 2 hours to complete, as a skilled mechanic burns away the soot particles in your DPF at a high temperature.

Important note: You should know that it is illegal in the UK to remove or replace your DPF yourself, and you won’t pass your MOT without one if your car was built with one fitted!


How Much Does a DPF Replacement Cost?

Ideally, you would take care of your DPF with both active and passive regeneration working as they should, and an investment in a DPF clean when needed. However, if a DPF replacement sneaks up on you, it can cost anything from £1,000 to £3,000!

To avoid the need for a replacement, you should keep tabs on your DPF warning light, as this will be the biggest indicator that your DPF is almost 70% full of soot. Once the soot level reaches 90%, there will be little you can do to prevent the need for a DPF replacement, as your car will have already entered a performance-limiting 'limp home' mode.


How Much Does a DPF Cleaning Service Cost?

At BookMyGarage, DPF cleaning prices at our network of UK garages start at just £99! This includes a full inspection, diagnosis and flush, all performed by skilled mechanics. In just a couple of hours, you can help your car to pass its next MOT and reduce your carbon footprint at the same time. We recommend that you book in for a DPF cleaning every 6 to 9 months, so that you can make sure you prevent the need for an expensive and avoidable DPF replacement.

Keep DPF problems at bay – book a DPF cleaning service today!


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