What is a DPF (Diesel Particulate Filter)?

What is a DPF (Diesel Particulate Filter)?


Since the Euro 5 regulatations that came into effect in 2009, Diesel particulate filters have effectively been mandatory on all new Diesel engined vehicles.  

The main aim of this regulation change was to reduce the amount of harmful pollutants emitted in towns and cities. In this article we’ll delve into what they are, why they’re used and what the drawbacks can be. 

What is a DPF? 

As the name suggests, it is a filter that prevents harmful particulates from exiting the engine via the exhaust system. The main particulate it stops is soot, this is a by-product of using diesel fuel.  

A DPF’s job is to trap the soot and hold it until it can be transformed into ash via a process called regeneration, which requires the DPF to be extremely hot.  

The main benefit in buring soot into ash is that the latter is much less harmful to humans, soot stays in the air we breathe for a long time, and makes up most of inner city air pollution. 

How does a DPF work? 

Inside the unit are a series of filters, these trap the soot and keep it until it burns away. As already mentioned the burning process is known as regeneration, and it requires the filter to be at nearly 500°C.  

As the exhaust only reaches these temperatures at high speeds and engine loads, the ash will typically not burn off until the car is driven at high speeds for an extended period, such as when on the motorway. 

This helps to both reduce the amount of soot emitted in and around town, but also to reduce the amount of overall polutants deposited into the air we breathe. 


There are two types of regeneration; Active and Passive. Passive regeneration is the type just explained, this happens during normal usage (as long as long journeys are made) and the car’s computer needs not intervene.  

Active or Forced Regeneration 

If for any reason the car is not driven at high speeds, for instance through traffic in town, the car's ECU (Electronic Control Unit) will detect that the DPF has not regenerated for a long time. It will then start an active regeneration cycle. 

Active or forced regeneration generally involves the engine altering the fuel/air mixture in order to make the engine produce more heat from the exhaust, this is not very fuel efficient but can help to remove built up soot. 

Some cars even use a specialst catalyst solution stored in a tank, that is injected into the DPF to aid the burning of deposited soot. 

What is a Blocked Diesel Particulate Filter? 

A common fault with any DPF system is the complete or almost complete blockage of the filter due to excessive soot build up.  

One of the most cited reasons for not choosing a diesel engined car is that they need to be driven long distances, this is in part due to the DPF, another common cause can be the EGR valve.

As already mentioned, the DPF needs to reach temperatures of approximately 500°C in order for the soot to be burnt off. If the vehicle is only driven for short periods of time, the passive regeneration of the DPF will not occur.  

The ECU will inevitably try to force a regeneration, but this can only be completed if the car is at a certain temperature already and will be driven for a certain time after the regeneration has started. This is often not the case within cities and with infrequent use.  

Does my car have a DPF? 

Diesel Particulate Filters are a relatively new addition to the average diesel vehicle in the UK, they have been used in off-road diesel vehicles since 1980, such as forklifts and excavators. HGVs and trains have also used them for many years. 

It wasn’t until the new Euro 5 regulations in 2009 that meant all diesel vehicles sold in Europe now required a filter to bring overall emissions down. Many manufacturers added them in advance, The PSA group (Citroen and Peugeot) were the first manufacturer to include DPFs as standard. 

This means if your diesel car was sold after 2009, it will almost certainly have a DPF. Many cars prior to 2009 will do as well though, as manufacturers modified their designs to comply in advance. 

The best way to check is to look in your owner’s handbook for reference to DPF regeration or similiar. 

How to avoid a clogged DPF 

There are a number of considerations when driving a vehicle with a DPF, to ensure it does not get prematurely clogged and cause issues. 

Don’t drive short distances 

As we discussed in the regeneration section, the exhaust needs to be hot to burn off the soot, if it never reaches a temperature high enough, it will only become more blocked. 

To mitigate this, don’t use the car for solely short journeys, or ensure at least once a week it does a journey above 40mph for over 30 minutes, or more if possible. 

Use the Correct Oil 

Special oils have been developed for use in DPF-fitted cars, with a lower content of elements that can cause soot build up, namely Sulphated Ash, Phosphorus and Sulphur. 

These are usually labelled as low SAPS, and will be found at any regular shop selling engine oil. For more information about oils, check out our guide here.

Take care of any other engine issues promptly 

Diesels are generally more complex than petrol engines, and have various components that can fail and cause issues with DPFs. 

You should pay close attention to any issues pertaining to: 

  • Stuck or faulty injectors 
  • Failing turbo seals 
  • EGR valve issues 

All of the above can cause excess oil or soot to exit the engine, either through miss-fueling or oil consumption. These will clog up a DPF in record time, so be on the lookout for dash warning lights and act quickly. 

How to tell if your DPF is blocked 

In order to prevent the need for complete DPF replacement, you should be aware of any issues as soon as they arise. Here are the most common symptoms of a blocked DPF: 

  • Poor running (low power, engine cannot rev freely) 
  • Issues starting 
  • Poor fuel economy 
  • Start-stop function not working 
  • Warning lights on dash 
  • Excessive smoke 

If your diesel is suffering from DPF issues, it's best to sort it as soon as possible, as it will only get worse. For interest free vehicle repairs and servicing, enter your vehicle registration and postcode on our website. 

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