If you own a diesel car, it’s likely that your car is fitted with a DPF or Diesel Particulate Filter. It’s also likely that you may not know exactly what a DPF is or how to look after it.

DPF’s have been fitted to diesel-fuel cars for almost 20 years. If they’re not maintained correctly there could be serious consequences.

Here we explain exactly how to look after them.

So what is a DPF?

A diesel particulate filter or DPF is a filter that captures exhaust soot in order to reduce emissions. DPF’s have a limited capacity, however, and so the trapped soot must be regularly emptied or ‘burned off’ to regenerate the DPF.

The regeneration burns off any excess soot deposited in the filter, reducing harmful exhaust emissions and helping to prevent black smoke, particularly when accelerating.

Did you know that it is illegal to remove a diesel particulate filter?

Owners of diesel vehicles face fines of up to £1,000 if caught having removed their diesel particulate filter. Removing your DPF can also invalidate your car insurance.

How do I tell if my DPF is blocked?

If the DPF becomes blocked with soot or a fault develops with it, an orange light will appear on the dashboard to indicate a problem.

What causes a DPF to get blocked in the first place?

  • Short journeys at low speeds are the main cause of blocked DPF’s.
  • Car manufacturers often go as far as recommending drivers choose a petrol car instead of diesel for those regular ‘short’ journeys.
  • Another cause of DPF failure includes poor servicing. A diesel particulate filter on a poorly serviced car is likely to fail sooner than a well maintained one. They should last for an estimated 100,000 miles if maintained correctly.
  • Choose the correct type of oil for your car – some motor oils contain additives that can actually block or damage your DPF.
  • Aftermarket performance modifications can damage a DPF, as can using low-quality fuel or running the car when the fuel level is low. At low fuel levels, your diesel car may avoid DPF regeneration in order to save fuel.

What’s the best way to maintain a DPF?

In order to maintain a DPF you need to make sure that it is fully capable of regeneration when it’s blocked with soot (or) when the warning light appears on your dashboard.

There are two types of DPF regeneration: passive and active.

Passive regeneration explained

Passive regeneration is active when the car is at speed on long motorway journeys. This allows the exhaust temperature to increase and burn off the excess soot in the DPF.

It is advised that drivers regularly give their diesel vehicle a good 30 to 60 minute run at sustained speed on a motorway or A-road as a result.

Active regeneration explained

Active regeneration occurs when extra fuel is injected automatically, by the vehicle’s ECU, when a filter reaches a predetermined limit. This raises the temperature of the exhaust and in turn, burns off the stored soot.

Problems can occur during short journeys, as the regeneration process may not finish. In this instance, the dashboard warning light will indicate that the filter is still blocked.

Driving for 10 minutes or more at speeds in excess of 40mph should make it possible to complete a regeneration cycle fully, and clear the dashboard warning light.

DPF active regeneration is taking place when the following symptoms occur:

  • A slight increase in fuel consumption
  • Deactivation of automatic Stop/Start
  • A hot, acrid smell from the exhaust

What if active or passive regeneration fails to work?

If the dashboard warning light persists or turns red, then do not leave it too long before getting the car looked at by a specialist. An inexpensive fix can become something much more expensive if left unchecked.

A blocked DPF can be cleaned by a garage, in a process called forced regeneration. This usually costs around £100 and is highly successful in removing the excess soot and allowing the DPF to automatically regenerate again.

You need a fully functional DPF to pass your MOT

A DPF check has been part of the MOT test since February 2014. If a filter has been removed, the car will fail its MOT. Removal of the DPF will also cause the dashboard warning light display – This is an instant MOT fail.

What are the cost implications of a new DPF?

DPF’s can be an expensive component. Your manufacturer could charge in excess of £1,000 depending on the make and model of your car.

It’s not uncommon for the cost of a replacement DPF to be more than the value of the car.

It is possible that third-party suppliers of diesel particulate filters could charge less, but tread with caution: they must still have the correct ‘Type Approval’ or they may not work properly, costing you more in repairs.