Diesel particulate filters explained

Date Posted 6th January 2020
Read Time 6 min read

If you own a diesel car, you’ll most likely have a diesel particulate filter (DPF).

Its goal is to reduce the number of emissions released by your diesel engine in an attempt to make it more environmentally friendly. 

They usually don’t require much attention, but if you don’t look after your diesel particulate filter or tamper with it, it can work out costly. 

To help you avoid this, we’ll answer some frequently asked questions and tell you everything you need to know. 

Firstly, the basics. 

What is a diesel particulate filter?

A diesel particulate filter, or DPF, is a filter that sits along your exhaust pipe, usually near the front, which intercepts soot to stop it entering the air we breathe and the wider atmosphere. 

If you think back to old-school diesel cars, you’d often see a cloud of black smoke leave the exhaust on start-up or acceleration. Basically, a diesel particulate filter prevents that. 

It’s designed to reduce the number of emissions that leave your vehicle’s engine by catching the soot then burning it off – known as regeneration. 

And although your diesel particulate filter self-regenerates, it’s dependant on you. Without your input, it can’t clean itself out, which can lead to blockage and the need for replacement.  

How long does a diesel particulate filter last? 

A diesel particulate filter – on a well-serviced car – should usually last more than 100,000 miles. 

However, if you don’t keep up with your vehicle’s servicing needs and keep your diesel particulate filter well maintained, you may have to replace it sooner, which can be very expensive. 

Why is my diesel particulate filter blocked? 

Usually, a diesel particulate filter becomes blocked because the car isn’t doing long enough journeys to regenerate. 

If you drive a diesel and only do short journeys around town, for example, you may find that your diesel particulate filter eventually clogs up – as your exhaust isn’t getting hot enough to burn off the excess soot. 

How do I know if I have a blocked diesel particulate filter? 

Most diesel cars nowadays are fitted with a warning light that will signal if your diesel particulate filter is blocked. It will look something like this. 

If that happens, you’ll need to unblock it by regenerating.

How to unblock and maintain a diesel particulate filter

Passive Regeneration and Active Regeneration

To unblock your diesel particulate filter, it needs to be hot so it can burn off any excess soot. 

If you regularly drive for long periods at sustained speeds on motorways or A roads, you should be doing enough for your diesel particulate filter to self-regenerate. 

It’s called passive regeneration and it’s the easiest way to maintain your diesel particulate filter. 

However, if you have a diesel vehicle and rarely reach decent speed for sustained periods of time, most diesel particulate filters will try to self-regenerate through what’s called active regeneration. 

Active Regeneration

Active regeneration is when your vehicle injects more fuel into its system to try and increase the exhaust’s temperature. 

For it to work, you should try to drive constantly for around 10-15 minutes at around 40mph or above. Anything less than that can cause the regeneration to fail and you may encounter further problems down the line. 

You can usually tell when your car’s onboard computer is trying to complete an active regeneration cycle as you’ll typically use more fuel, hear the cooling fans running, won’t be able to use the start/stop function, and you may smell a burning smell from your exhaust. 

Other signs include a change to engine tone and increased idle speed.  

If you notice this happening, it’s important you let your vehicle complete its regeneration cycle. You’ll be able to tell when it’s been successful as the diesel particulate filter warning light will go off.

My Diesel Particulate Filter Warning Light Won’t Go Off

If you’ve tried both passive and active regeneration and your vehicle’s diesel particulate filter warning light is still on, you should get in touch with your local service centre to have it looked at.

Usually, if you act quickly the cost isn’t that high. But the longer you leave it, the more harm you can cause, which could lead to you needing a completely new diesel particulate filter.

How much does a diesel particulate filter cost?

A new diesel particulate filter can be costly – from around £1,800, in fact. That’s why it’s important you maintain yours where you can. 

It’s also worth noting that the older or bigger the car, usually, the more expensive it is to replace. 

You can find them cheaper online, but it’s important to be wary of the seller. Diesel particulate filters are not universal, so you need to make sure it’s the right one for your vehicle and that its quality is assured.

Failing to do so can create even greater problems resulting in a bigger bill. 

Can I remove my diesel particulate filter?

Driving a vehicle which is meant to have a diesel particulate filter without one is against the law. 

If you get caught, you’ll face a £2,500 fine. Your vehicle will also fail its MOT if you remove or tamper with the diesel particulate filter.

Thousands of motorists have been fined for driving without a diesel particulate filter – when in fact, in most cases, it’s cheaper just to get it replaced. 

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