DPF exhaust filter

What is DPF on a car, its function, and how best to maintain it?

There are plenty of technical terms and car part names bandied around the motor industry that leave many drivers in confusion.

DPF is one such term.

So, what does DPF mean? Well, DPF stands for diesel particulate filter. They feature solely in diesel cars and were introduced in 2009 to prevent soot, or particulate matter, given its correct name, from entering the atmosphere.

Diesel cars are renowned for producing a lot of this type of soot. It’s a contributor to respiratory problems and cardiovascular disease, so preventing excessive amounts from entering the air that we breathe, is essential.

New Euro 5 exhaust emissions legislation was introduced at that time to lower CO2 emissions and made DPF use mandatory. All new diesel cars manufactured after that time had to include a DPF.

What is DPF regeneration?

Diesel engines burn fuel differently to petrol engines. The soot in question is very fine and almost invisible, yet still causes a great deal of harm. So, a method of eliminating it needed to be introduced.

Collecting the soot is the first part of the process. The second part is emptying the soot at regular intervals. This happens by burning it off at extremely high temperatures, which leaves a tiny ash residue.

This ash can’t be removed without specialist cleaning, so, fortunately, a DPF should last well over 100k miles before that needs to happen.

Burning off the soot is known as regeneration, and it happens in 2 ways.

DPF passive regeneration

Passive regeneration was designed to actively burn off the soot during some instances of regular driving.

When the exhaust temperature increases to a high enough level (around 500°C), the DPF can cleanly burn off the excess soot in its filter. However, this only happens at speeds of over 40mph and needs to carry on for around 15 minutes.

This created a problem for town and city drivers, whose short journeys consisted of stop-start driving at slower speeds. For the drivers who rarely frequented the A Roads and motorways, where passive regeneration could happen automatically, there needed to be another option.

What prevents passive regeneration?

  • Short journeys where the engine can’t get up to temperature
  • Stop-start driving
  • Low fuel levels
  • The wrong type of engine oil
  • Excessive mileage between services
  • Problems with the fuel system
  • Issues with the Exhaust Gas Recirculation system

DPF active regeneration

Active regeneration happens when extra fuel is injected automatically as the filter reaches an approximately 45% full limit. The extra fuel raises the temperature of the exhaust and burns off the stored soot.

Yet the journey has to be long enough to complete a full cycle. The drivers whose journeys rarely amount to the necessary time it takes are likely to see the DPF warning light illuminate on their dashboard.

Things you might notice during active regeneration:

  • Cooling fans will run
  • The engine will idle at faster speeds
  • Auto stop-start is turned off
  • Fuel consumption will rise
  • A hot, acrid smell emitting from the exhaust
  • The car runs with a different engine sound

What will a clogged DPF do?

When a driver sees a DPF warning light, they know its time to get out for a good 15-minute blast on the motorway or open road. Otherwise, they’re risking a problematic build-up that can lead to serious problems.

It’s a good idea to understand the best way to regenerate your car’s DPF. In some cars, stopping after the warning light illuminates can make the blockage worse. Ideally, you’d be wise to run it and regenerate the DPF as soon as you see the warning light come on.

What does DPF full mean?

If you don’t keep up with sensible DPF maintenance, your filter will fill up beyond an acceptable level. At this point, your car will illuminate a DPF full, or equivalent warning light, and enter into limp mode.

What happens when a DPF is blocked?

To have a dealer empty your DPF or force a filter regeneration will cost around £100. However, in the more severe cases, your DPF will need to be replaced; a process that can run into costs of thousands of pounds.

What is a DPF on a car likely to cost to replace?

A new DPF can cost between £1k and £3,500. Conscientious drivers choosing diesel motoring to save money will lose anything they’ve saved with such an imposing repair bill.

Given that the lifespan of a well-maintained DPF should be well over 100k miles, there’s a high chance that it should outlive the car it’s in. Replacing such a costly part when the value of the car is likely near its lowest, will probably dictate the end of its useful life.

DPF additives

Generally, the DPF’s location is close to the engine where the exhaust is hottest. The temperature provided in this location gives the best chance of passive regeneration.

In some car engines, often where the DPF is seated in a different location, there needs to be a fuel additive to lower the ignition temperature of the soot particles. This is so regeneration can happen at the lower temperature.

The additive (Eolys™ fluid) is stored in a separate tank and automatically mixes with the diesel fuel. A full tank of additive costs around £200, including fluid and labour costs. It should last a good 70k miles.

If you get a warning light indicating that the fuel additive is low and needs refilling, see to it as soon as possible. Without the additive, your DPF will quickly become blocked.

It is illegal to remove your car’s DPF

Your DPF is fitted to conform to European emissions regulations, so its removal is against the law.

Driving without a DPF in a car manufactured after 2009 will fail an MOT and could also invalidate your insurance cover.