Diesel Particulate Filters: All You Need to Know

If you own a diesel car manufactured after 2009, it will feature a device called a Diesel Particulate Filter (DPF). Introduced by carmakers to help reduce exhaust emissions, DPFs unfortunately have an obvious flaw in that they can quickly become blocked, resulting in the hassle of a breakdown and a potentially costly repair bill. 

While you’d expect it would be difficult to block the exhaust DPF, it’s actually a lot easier than you might think with normal driving. Frequent short journeys, the wrong engine oil or problems with other engine components can contribute to a blocked DPF — and the problem is thought to have affected thousands of drivers since they were introduced.

To help you avoid the hassle of a blocked DPF, here we offer further information on what it is and how you can prevent the problem.

What is a DPF?

dpf diesel

The Diesel Particulate Filter was introduced to help diesel cars pass increasingly tough emissions tests. Its job is to trap soot particles produced through the burning of diesel, and prevent them from entering the atmosphere. Like any filter, DPFs become clogged after so many miles, and then require cleaning.

What is DPF regeneration?

Rather than cleaning the DPF, the term used by professionals is ‘DPF regeneration’ as, technically, the soot particles are burnt off from the filter to ‘regenerate’ it. This usually happens automatically when the engine reaches a certain temperature — normally at motorway speed, or during a long drive if you’re driving more slowly. This is referred to as passive DPF regeneration as neither the driver nor the car does anything out of the ordinary – it just happens.

Why do they go wrong?

If a car spends most of its time being driven around town on short start-stop journeys, the exhaust doesn’t get up to temperature so the soot doesn’t get burnt off and the DPF does not ‘regenerate’ on its own.

car exhaust dpf diesel

Thankfully, the engine monitors the DPF and when its starts to get blocked, it will alert the driver; this is normally represented as a dashboard warning light. If the DPF warning light comes on it’s because the engine management system needs to ‘actively’ regenerate the DPF. You should take your car on a run to allow the regeneration process to take place. Most engine management systems will actively regenerate the DPF when the car is driven at speeds above 40mph for around 10 to 15 minutes. The soot particles trapped in the filter will soon burn away and the warning light should switch off. Just make sure you drive within the speed limit as the last thing you need is a speeding ticket!

How to tell if my DPF is blocked?

Aside from a DPF warning light on the dashboard, there are plenty of tell-tale signs that there’s a blockage in the system. Some of the warning signs to watch out for include:

  • Reduced engine power
  • Lack of revs when you press the accelerator
  • Marked reduction in fuel economy (expressed by real-time MPG figures)
  • Diesel odour in the cabin
  • Dark smoke from the exhaust when accelerating
  • Ignition problems

Remember: if your DPF is blocked, one of the first signs is a warning light on the dashboard. The symptoms of a blocked DPF are similar to other faults, so if you’re not seeing a DPF warning light, we’d recommend taking your car to a professional mechanic to diagnose the issue.

What happens if my car breaks down due to a blocked DPF?

If the dashboard warning light is ignored or the DPF is not regenerated by taking the car on a motorway for a good run out, it can become more blocked and the engine will eventually go into ‘safe’ or ‘limp’ mode. At this point, you will most likely see a new dash warning saying that the car should be taken to a garage resulting in a repair which could be expensive.

exhaust check emissions
The garage may be able to force the DPF regeneration by connecting the engine management system to a diagnostics machine. This typically costs around £250, but can also damage the DPF through thermal shock. Replacement DPFs cost around £1,000, so it’s really not worth letting yours become clogged, no matter how late you are for work.

And don’t think you can have the DPF filter removed to save cash, either. While it is technically possible, and the car would continue to run as normal, a missing DPF filter will result in an automatic MOT failure, and could invalidate the car’s warranty, leading to more expensive repair bills.

What is ‘active’ regeneration?

Recognising that many diesel cars don’t get the right kind of use to make high-speed DPF regeneration a reliable option, some car manufacturers have turned to ‘active’ regeneration to solve the issue of a clogged particulate filter. This is when engine control software monitors when the DPF is becoming blocked, and starts a process in which more fuel is injected into the engine to increase the temperature, triggering regeneration.

Active regeneration takes around 5 to 10 minutes to complete, and normally happens every 300 miles or so depending on how you drive and how often you use your car. When active regeneration is taking place, you may notice a change in how the car sounds and feels – this is due to increased power to the cooling fans and a faster engine idling speed.

You might also find that the automatic start/stop function doesn’t work, or that there’s a strong smell coming from the exhaust. This is normal, and your car should return to normal after the cycle is finished.

Active regeneration isn’t a fool-proof solution, however. If you only ever make quick trips around town in your car, it can be easy to interrupt the process. Switching your car off while it’s halfway through active regeneration can cause problems, and may mean the filter hasn’t been properly unclogged.

What is ‘passive’ regeneration?

Unlike active regeneration, ‘passive’ regeneration occurs naturally during everyday driving. If, for example, you spend a lot of time driving on the motorway, your car’s DPF should stay clear due to the process of passive regeneration – wherein the heat of the exhaust emissions is high enough to prevent the filter from becoming blocked.

The main difference between passive and active DPF regeneration, then, is that active regeneration is triggered by the ECU when sensors detect that the DPF is getting clogged, while passive regeneration happens when the correct speeds are maintained for a long enough period to keep the filter clear during day-to-day driving.

DPF filter issues can be an annoying problem for diesel car drivers, so it’s easy to see why someone would want to remove the filter completely to stop it causing issues. However, as DPFs remove so many harmful particles from engine emissions, you could face significant penalties for cutting the DPF out of your car’s exhaust system.

DPFs are fitted to diesel cars to meet European emissions regulations, so it’s actually an offence to remove them and you could be fined under the Road Vehicles (Construction and Use) Regulations Act. It can also invalidate your insurance and cause you to fail your MOT. Previously, removing your DPF meant instant MOT failure, but under changes brought in in 2018 you can fail if it’s still in place but you’ve tampered with it. It basically needs to be intact and working or you’ll fail your MOT.

So, while you might think removing your car’s DPF will save money on potential repair bills, the consequences will almost certainly cost you more in the long term.

Is there another way to look after the DPF?

For years after DPFs were first introduced, the only way to clean them was to take long drives every 300 miles or so – a pain for those that do their driving in town or when the vehicle is used for commercial use such as a taxi or van. Now there is another way to keep your car’s DPF clean and clog free, and it’s all thanks to Redex DPF Cleaner.

Up to 30% more efficient than other products on the market, Redex DPF Cleaner works by reducing the temperature at which the soot burns off the filter. No need for any more inconvenient drives, you can just drive normally. One bottle of our fast-acting formula is enough to put out the DPF warning light, so you can enjoy complete peace of mind behind the wheel.

How long can you drive with the DPF light on?

The short answer is: not very long!

If you see a DPF warning light, it means that your car’s filter is almost at capacity. Continue to drive at low speeds without attempting to clear the blockage, and you could begin seeing some of the symptoms listed above.

You should, however, have enough time to reach a motorway or A-road, where driving at a faster speed for around 15-20 minutes should help to clear the blockage. Alternatively, of course, you can pull over right away and use Redex DPF Cleaner to clear the blockage without the need for an inefficient and inconvenient faster drive.

Do I need a diesel particulate filter to pass the MOT?

Categorically, yes. If you’ve had your DPF removed your car will fail its MOT. DPF checks have been part of the annual MOT since 2014.

What is the cost of a new DPF?

There’s no getting around it: replacing a DPF is expensive. If you need to buy a new one, it could set you back £2,000-£5,000 depending on the make and model of your car.

The good news is that provided you keep on top of clearing your car’s DPF and, obviously, don’t remove it, you should never require a replacement.

So there you have it: a complete guide to diesel particulate filters. For more diesel-related car maintenance advice, be sure to check out the Redex blog and advice pages. Alternatively, tap here for full range of diesel fuel additives.