DPF Light (What Diesel Drivers Need To Know)

By: John Tucker

The DPF light comes on to tell you that there’s a problem with your car’s diesel particulate filter. This is something that has been fitted to diesel cars since 2009 to reduce the amount of soot released through the car’s exhaust system, lowering emissions. This soot is harmful and can cause respiratory problems.

Although having one of these filters in your car’s exhaust helps to significantly reduce the amount of soot emanating from your exhaust, it can become blocked, causing the DPF warning light to become illuminated on your dashboard.

What causes the DPF to become blocked?

When the deposits of soot build to a certain level, the car’s engine control unit starts what is known as ‘passive regeneration’. More fuel is injected into the engine, which raises the temperature in the exhaust system. This burns the soot, turning it into a fine ash and expels it through the exhaust.

Your car usually performs this process whilst travelling at higher speeds, as this is required to maintain a high temperature in the exhaust system.

However, if you spend most of your time doing small trips around town, rarely reaching motorway speed, your car cannot perform this regeneration process. This causes the soot to continue to build up in the DPF, eventually causing the diesel particulate filter light to become illuminated.

dpf light

What do you do when you see the DPF light?

When the DPF light has become illuminated, it’s telling you that ‘passive regeneration’ has failed and that you need to actively regenerate the diesel particulate filter.

You can do this by increasing your speed to more than 40mph for between 10 and 15 minutes. This should allow the exhaust to reach sufficiently high temperatures to burn the soot and clear the filter.

During the regeneration, you may notice things like your engine making an unusual noise, a slightly pungent smell coming from the exhaust and an increase in your fuel consumption. This should all return to normal once the active regeneration process has completed. You should also see the DPF light go out.

If you cannot maintain a speed of more than 40mph for long enough to allow the regeneration process to complete, the soot will continue to build up in your filter and your DPF warning light will remain on until you have the problem fixed.

Call us for advice

Can you ignore the DPF light and carry on driving?

Technically yes, but we would not recommend it. If you ignore the DPF light and continue driving, the build-up of soot will soon reach a point whereby your car has to enter ‘limp-home’ mode in order to prevent any damage to the engine.

This will probably result in other warning lights becoming illuminated, advising you to take your car to your local garage for diagnosis.

Your garage should be able to perform a forced DPF regeneration, which should clear the filter and allow it to function properly once more. However, it’s not a guaranteed fix, especially if the filter itself has become damaged or other damage has been done to the engine. More serious repairs could run into the thousands, depending on the damage and the vehicle.

It really isn’t worth ignoring the DPF light.

Can DPF problems be avoided?

Only by owning a car that suits your needs. If you drive long distances, perhaps commuting to work, then a diesel with a particulate filter would be fine. You would regularly reach speeds required to perform both passive and active regenerations, reducing the likelihood of having any problems with the DPF.

However, if you only do short journeys, perhaps a diesel isn’t really for you. If you only use your car to pop to the shops, or do the school run, a petrol engine is more suited to you. If you ran a diesel with a DPF, you’d rarely give it the opportunity to clear its DPF, resulting in you seeing the DPF light more than you would like.

If you would like to find out about other warning lights, make sure you read our Dashboard Warning Lights Explained guide.

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