What is a DPF (Diesel Particulate Filter)?

Sometimes referred to as a soot filter, a DPF traps the soot from the exhaust in order to reduce the harmful emissions from diesel cars. Euro 5 emissions standards made a DPF mandatory for all new diesel cars from 2009, and it’s illegal to remove it. Remember the black smoke you used to see coming out from diesels when they accelerated? Those are the kind of emissions the diesel particulate filter was made to capture.

DPFs will fill up over time, so they need to be periodically regenerated to keep them working properly. DPF regeneration is done by burning off the soot at a high temperature, reducing it to finer ash. This ash is then expelled from the system. Burning off the soot like this prevents it from building up, blocking the filter and stopping the engine from running.

Most diesel particulate filter problems occur when the regeneration process is interrupted. If you use the car mostly for short journeys at low speeds (the school run perhaps, or rush-hour commuting), the DPF might not get a chance to burn off the build-up of soot, and the filter can become clogged. For this reason, petrol engines are likely to be a better option for drivers using the car mostly for short city hops. Using the wrong kind of oil can also cause problems, as some contain additives that can block the filter. Properly serviced, a DPF should last at least 100,000 miles, but poor servicing can reduce its lifespan.

Look on your dashboard. Can you see an orange light that looks like an exhaust pipe with 11 dots in the middle? If so, your DPF is getting clogged or developing a fault. If this light doesn’t disappear, turns red, or is joined by other warning lights, you need to get it checked out before a cheap fix turns into a much more expensive one.

There are two types of regeneration: passive and active. Your car should take care of both these by itself if driven fast enough for long enough.

Make sure you’re using the right kind of oil for a DPF – you need one of the SAPS (Sulphated Ash, Phosphorous, Sulphur) oils. Using regular oil could block the filter. It’s also recommended to keep the tank at least a quarter full – if you’re running on fumes, the car may interrupt DPF regeneration to conserve fuel. Also, make sure you’re giving the car a decent service – with proper servicing, a DPF should last at least 100,000 miles.

If you don’t take the car out for a higher-speed run and instead continue your normal driving with the warning light on, other warning lights will come on. The car may eventually go into ‘limp home’ mode in order to limit damage to the engine.

If it’s left for too long, the DPF won’t be able to regenerate itself and you’ll need to take it to a garage. The cheapest fix is a DPF cleaning process called forced regeneration, which should hopefully remove the excess soot for not much more than £100. If that doesn’t work then you’re probably looking at a replacement DPF and a bill running into the low thousands. This bill is unlikely to be covered by manufacturers’ warranties if it’s deemed to have been caused by your driving habits rather than a defect in the filter.

Think about how you’ll be using the car. If you’ll be doing a lot of fast driving, you’re unlikely to run into any problems with a DPF – just follow the recommendations in the car’s handbook and don’t ignore any warning lights. If you’re buying used, the DPF is a potential hidden cost to be aware of. If a car is approaching 100,000 miles on the clock and its DPF has never been replaced, it’s likely nearing the end of its lifetime, so the cost of a new one needs to be factored into your purchasing decision. You should make sure it hasn’t been illegally removed too. All cars designed to have a DPF are now checked for that in the MOT test – if it’s not there, you’ll fail immediately.

The diesel particulate filter (DPF) should do its job quietly and unobtrusively. Make sure it keeps doing that by giving your diesel car the chance to stretch its legs now and again. And, if you want to avoid a minor issue turning into a major expense, don’t ignore the warning light.

Looking for a new diesel? Take a look at the ten best diesel cars on the market today.