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What is a Diesel Particulate Filter?

If you drive a diesel car or are looking to purchase one, it is vital to know about the Diesel Particulate Filter (DPF).  We explain what it is, how it works and most importantly how to look after it.

What is a DPF?

A Diesel Particulate Filter is a filter that captures and stores exhaust soot. Since diesel cars produce soot they are therefore fitted with a DPF. The DPF is designed to specifically collect the soot that is created when the diesel is burned, this prevents harmful substances from entering the environment and reduces the emissions from diesel cars. The soot created by the process of combustion which is collected in the filter is burnt off to make sure that the filter does not become blocked and that it can carry on trapping more soot. This is know as DPF regeneration. Problems start to occur once the soot accumulates as this blocks the filter and stops the engine from working. DPFs became standard in 2009, but some diesel cars were fitted with this technology prior to this date.

How does the DPF work?

The DPF’s job is to trap the particles, so when they become clogged up with soot if blocks the DPF. A DPF has to be emptied regularly in order for it to work at its optimum level.

The car is however built to clean the filter by heating up the trapped particles, which then turns into ash and is released. However, this is only possible when the car is travelling on the motorway, at higher speeds. When you drive for a long time at high speeds, the exhaust reaches very high temperatures. Driving at 70mph or above for a while will heat the exhaust enough to burn off the soot particles which have been trapped by the DPF. This is known as Passive DPF Regeneration.

Many drivers don’t drive long distances on the motorway regularly, so car manufacturers had to come up with a different way to clear the DPF of soot – Active DPF Regeneration. Active DPF regeneration is activated when the amount of soot gathered in the DPF reaches a certain level. In most cars, this level is set at approximately 45% of the DPF’s total capacity. 

What happens when a DPF fails?

Generally a DPF warning light will appear on your dashboard (check your owners manual to see what the DPF warning light looks like). While your vehicle won’t stop running the moment the light turns on, continuous driving of your car can lead to issues like poor fuel efficiency, reduced performance, and the build-up of soot will soon reach a point where your car enters ‘limp-home’ mode to prevent any damage to the engine. An illuminated DPF light will mean an MOT failure.

The most common reasons for DPF failures are:

  • Faulty fuel injectors sending too much fuel to the fuel/air mixture
  • Short journeys and stop/start traffic cause the DPF to get blocked as the trapped particulates are not able to reach the temperature required to burn the soot.
  • A high mileage car – as it will find regeneration harder.
  • Using incorrect engine oil
Mechanic's hand under bonnet wearing a grey glove and changing the oil filter

How do I look after my cars DPF?

The best way to look after your vehicles DPF is to ensure it is fully able to regenerate itself when the soot accumulates. Passive regeneration takes place when the vehicle is travelling at a higher speed. To ensure that the filter is cleared, it is suggested that every few hundred miles the vehicle is driven for around 15-20 minutes at a consistent high speed on the motorway, ensuring this is done regularly should clear the filter. As the exhaust temperature increases it burns off the excess soot in the filter.

The main cause of blocked DPF is short journeys at low speeds. Hence, why it is recommended to opt for a petrol car if your journeys consist more of short local drives.

It is important to use the right oil for your vehicle, as some oils contain substances that can block the DPF. High-quality premium fuel includes additives that help burn off particles, and can help in clearing the DPF.

How much will it cost to clean or replace the DPF?

A franchised dealer or a reputable garage may be able to clean the units at a charge of around £120 + VAT. If, however, the DPF cannot be cleaned a replacement unit typically costs around £1000.

A failure to respond to the DPF warning lights will result in the engine losing power and stopping altogether. The DPF will then need to be replaced and the repair bill is likely to add up. Most DPFs will last for around 100,000 miles, but well looked after DPFs can last longer.

Top 5 Diesel Particulate Filter Repairs

Make Model Amount Paid
Dodge Journey £1,057.74
BMW 5 Series £980.20
Volkswagen Touareg £906.62
Jaguar X-Type £895.36
Jaguar S-Type £867.94

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