What is a DPF and What Does it Do?

A DPF is part of your exhaust system, found only in diesel cars. Diesel doesn't combust in the same way as petrol, so your car would produce more harmful emissions if the DPF didn't trap them as soot. It makes your car more environmentally friendly and helps it meet several EU emissions standards. Diesels naturally produce less CO2 (Carbon Dioxide) than petrol cars. This is one of the main gases affecting Climate Change, so driving a diesel might seem like a good way to reduce your Carbon Footprint. Unfortunately, it's not that simple. Diesels also produce high levels of NOx (Nitrogen Oxides). This is what the DPF traps - but there can be serious problems if the DPF stops working or can't complete what's known as 'regeneration'. This usually happens if you drive lots of short, low-speed journeys, or stop and start often. The DPF is also important in terms of your MOT. There has been a 240% rise in diesel cars failing their MOT emissions test over the last five years, mainly because of clogged or failing Diesel Particulate Filters. All of this can be avoided by the regeneration process - but how does that work?  


What is DPF Regeneration?

Over time, the soot will build up in your DPF, clogging it and reducing your car's performance. That's where DPF regeneration kicks in. This process burns off the trapped soot as cleanly as possible, and there are two forms. Active Regeneration happens around every 300 miles, and is initiated by the car's ECU (Electronic Control Unit) once the soot level reaches 40-45% capacity. The onboard computers inject extra fuel into the exhaust, which raises the exhaust temperature and burns the soot away within 5-10 minutes. During Active Regeneration, you may notice the following symptoms:

  • Cooling fans running
  • Faster engine idle speed
  • Automatic Start/Stop technology doesn't work
  • Increased fuel consumption
  • A hot acrid smell from the exhaust
  • The engine sounds different

These mean that the process is working and shouldn't worry you. Passive Regeneration happens when you drive at high-speeds, such as on a motorway. The exhaust temperature is naturally higher on these roads, and so begins a chemical reaction to neutralise the soot. This leaves only a thin ashy residue behind.  

Heavily smoking exhaust with black smoke pouring into the atmosphere

If your exhaust smoke looks like this, there's a good chance that your Diesel Particulate Filter isn't regenerating properly!

There can be problems with Active Regeneration. If the process isn't completed, and your car isn't undergoing Passive Regeneration often enough, your DPF will only get more clogged. Your car will struggle to complete Active Regeneration if:

  • Your engine management light is illuminated
  • There is less than 20 litres of fuel in the fuel tank, or your fuel light is on
  • The pressure sensors aren't working properly, or the pipes are damaged
  • Your Exhaust Gas Recirculation (EGR) system isn't working properly
  • Your vehicle is using the wrong engine oil
  • The Eolys level is too low

Make sure you check your vehicle handbook to use the correct engine oil, and keep the fuel tank well topped up. The last thing your diesel car needs is a lack of DPF regeneration.  

What are the Symptoms of a Blocked DPF Filter?

There is an amber DPF warning light, which looks like this.  

Diesel particulate filter warning light

This is the most common DPF warning light. If you see this, or any variants, you need to book a DPF clean as soon as possible!

  This always confirms a problem, but other warning signs arrive first. These include:

  • A loss of performance or power
  • The DPF regeneration process is constantly failing or not finishing
  • A rising oil level
  • Higher fuel consumption
  • Heavy smoke
  • The smell of diesel in the car

These indicate that your car needs urgent attention. If you're struggling to engage regeneration on your own, you might be wondering what you can do to avoid a costly repair bill. Your best solution is to invest in either a DPF cleaner, or a high-quality DPF clean booked through BookMyGarage. So, what are these options?  

What is DPF Cleaner?

DPF cleaner is a mixture of chemicals which remove soot and ash that has built up in the DPF. It flushes out the insides of your car when regeneration cannot be completed. There are a range of options when it comes to choosing a DPF cleaner. Here are some of the best DPF cleaner options for 2021, as recommended by industry experts. As you'll see, they're designed for regular use and are relatively cheap options to keep your DPF free from soot and ash. For a filter that hasn't been cleaned in a while and is significantly blocked, a more thorough deep clean is required. That's where DPF cleaning booked through BookMyGarage comes in! The process helps improve your MPG (Miles Per Gallon), regain performance and make your car cleaner and greener. As well as being beneficial for your DPF, it's also very affordable.  

How Much Does DPF Cleaning Cost?

Prices start from just £99 when booked through BookMyGarage. This includes a full inspection, diagnosis and flush carried out by highly-qualified mechanics. When you book a DPF clean, you reduce your chances of failing your next MOT. No one wants the stress and hassle of dealing with an MOT failure, especially for something as preventable as a blocked DPF. DPF cleaning only takes a couple of hours to complete, and leaves your car running like new. That way, you can reduce your carbon footprint as well! Most importantly, a DPF clean helps you avoid the eye-watering expense of a DPF replacement.  


How Much Does a DPF Replacement Cost?

A DPF replacement is incredibly expensive, and you should do all you can to avoid reaching this point. For a standard family car, a new DPF can cost up to £1,000, increasing to £3,000 for high-performance vehicles! This is only necessary once your filter is 90% full of soot. At this point, your car will have entered 'limp home' mode. It typically engages 'limp home' mode when your DPF is around 70% full of soot. This performance limiting mode switches off everything but the most essential features and limits your speed to a maximum of 35 - 45 mph. For your safety, your car will never cut the engine altogether. The DPF warning light will change just before it reaches 70% capacity to alert you. To avoid this cost and stress, we recommend booking a DPF clean every 6-9 months. Proper maintenance is much more cost-effective, and also helps this part last longer!  

Can I Remove the Filter Instead?

You might think that removing the DPF will solve many of these problems, especially if you're struggling with the regeneration process. In fact, it will only cause you more problems! You should NOT remove your filter. It is illegal and, as it has been tested as part of the MOT since February 2014, brings about an MOT failure if your car is found to not have one. It also brings a hefty fine - £1,000 for cars and £2,500 for vans! You shouldn't consider a DPF removal. There are other more simple changes you can make first - such as considering whether you should own a diesel car in 2021.  

A close up look at the exhaust system, with a blue and white background

It's not just your DPF that you shouldn't remove - your car always needs a functioning exhaust system! Tampering with any part of this system brings a fine and immediate MOT failure.

  If your filter is struggling to work at all, or your car has suffered a DPF related breakdown, your only option may be to replace the part altogether. So what is that, and how much does it cost?  


How Long Should a Diesel Particulate Filter Last?

A DPF should last at least 100,000 miles, if properly cared for. This includes regular car servicing, regeneration and cleaning. What's more, the ashy residue left in the filter doesn't build up or cause problems like excess soot. So, as long as you look after your DPF properly, you could get even more than 100,000 miles out of the part! If you're struggling with any of the problems mentioned in this article, there are several things you can do to improve your Diesel Particulate Filter. One of the best ways is by booking DPF cleaning through BookMyGarage. However, there are other options, including things you can do as part of your day-to-day driving.  

diesel cars and lorries driving on a motorway to improve DPF performance

One of the best ways to improve your Diesel Particulate Filter is regular motorway driving.


How to Improve Your DPF Filter

The best thing you can do for your car is to get it regenerating on a regular basis. You shouldn't panic while the car still works, even if the DPF warning light switches on. The best way to engage Passive Regeneration is by driving over 40mph for about 10-15 minutes. If you can drive at these speeds for longer and more frequently, the better. Diesel cars don't like low-speed, stop/start journeys, so the more motorway driving they do, the better it is for you! You should always know what to do before you see the warning light. Stopping the car to check your vehicle handbook can make the problem worse, especially if the car is close to entering 'limp home' mode. You might not be able to switch the engine back on if you switch it off!  


Your Diesel Particulate Filter is vital to maintaining a healthy diesel car. Here are the three most important things to remember about it:

  1. The DPF is necessary for meeting certain EU emissions standards, so can't be removed or tampered with in any way. Doing so brings a large fine.
  2. Active and Passive Regeneration are crucial to keep your filter in top condition. Your car will try and engage these on its own, but your driving habits have to help.
  3. If you need to clear your filter out, either go for an extended drive on the motorway or book a DPF clean through BookMyGarage today!