What is a DPF?

16th Mar 2021

By Paige Hodgkinson

How does a DPF work?

Diesel Particulate Filters (DPFs) remove Particulate Matter (PM) from diesel exhaust gas. PM is responsible for the black smoke emitted from the exhaust of diesel engines and contributes to air pollution which raises concerns with regards to human health.

A DPF is an after-treatment device that filters soot, containing PM, from the diesel exhaust system. Soot is produced when diesel engines burn fuel, but a DPF captures this soot to prevent the release of harmful exhaust emissions.

Once the DPF has captured the soot it stores it until the engine reaches a high enough temperature to burn the excess soot to ash, this is known as regeneration. There are two types of DPF regeneration; passive and active.

What is passive regeneration?


Passive regeneration happens whilst driving, using the heat of the exhaust. For passive regeneration to take place the car needs to have been running at a moderate to high speed for a decent amount of time in order to create the correct temperature for the soot to be burned.

Usually, passive regeneration requires 30 to 45 minutes on a motorway or A-road maintaining a continuous speed.

If you regularly drive short journeys or sit in a lot of traffic, passive regeneration will be difficult to achieve and may result in the DPF warning light appearing on your dashboard. Many cars are now designed with engine control software that senses when the DPF is getting blocked and triggers active regeneration.

What is active regeneration?

40 Road Sign

Active regeneration was designed for when passive regeneration isn't possible. This method is initiated around every 300 miles and works by injecting extra fuel to increase the exhaust gas temperature in order to burn the soot stored in the DPF.

A journey of a reasonable length is still required to complete the active regeneration cycle, usually 10 minutes at a speed of more than 40mph is suggested.

It's important to make sure the regeneration cycle finishes before turning off the engine otherwise the DPF can end up blocked with soot resulting in the DPF warning light illuminating on the dashboard.

Why is the DPF warning light illuminated on the dashboard?

DPF Warning Light

Due to the DPF storing soot it's common for it to get clogged, resulting in the DPF warning light being displayed on your car's dashboard. Don't let this light alarm you as usually it's simple to resolve on your own.

As mentioned above, regeneration burns the excess soot stored in the DPF to ash. Therefore, regeneration is often the resolution to the DPF warning light.

However, if the warning light remains illuminated, turns red or your car enters limp mode; there may be a more severe issue with your DPF and you should get this checked out urgently at your nearest Evans Halshaw retailer.

If ignored your DPF could become damaged resulting in costly repairs. A blocked DPF can be cleared by a technician using a process called forced regeneration which is usually successful.

How do you know when the DPF is regenerating?


There are a few signs to look out for which signal when the DPF is regenerating:

  • Start-stop will more than likely be deactivated
  • Cooling fan is running
  • Higher fuel consumption
  • A hot and unusual smell
  • Engine idle speed can be higher than normal

What to do if you're still having issues with your DPF

Even though a DPF can be frustrating if you're a driver that often does short journeys or spends a lot of time sat in traffic, they have been a part of the MOT since February 2014 and therefore your car will fail its test if you have it removed.

Hopefully our blog has answered your questions so you can now easily avoid DPF issues and the dreaded DPF warning light on your car's dashboard.

However, if you're experiencing issues with your DPF you can book your vehicle in for a free vehicle health check at your local Evans Halshaw retailer and our fully trained technicians will be able to take a look.