Diesels produce lots of soot (particulate matter) that can cause respiratory problems and contribute to the risk of cardiovascular diseases.
Modern diesel cars (since 2009) have to be fitted with a Diesel Particulate Filter (DPF) in the exhaust to stop this soot passing into the atmosphere.
The aim is an 80% cut in particle emissions but the technology's not without problems and our patrols are often called to cars with a blocked DPF.
To maintain performance a DPF has to be emptied regularly. This is usually done passively in a process called 'regeneration': when the exhaust temperature’s high enough, on motorways or fast A-roads.
- The collected soot is burnt off, leaving only a tiny ash residue.
- The ash can’t be removed – unless the DPF is removed from the vehicle and sent away for specialist cleaning – but a DPF in a car used correctly should be good for well over 100,000 miles.
Many cars don't get the right sort of use for passive regeneration to work so car manufacturers build in ‘active’ regeneration where the engine control software senses that the filter’s getting blocked and injects extra fuel into the engine to raise the exhaust temperature and trigger regeneration.
Active regeneration will be initiated every 300 miles or so depending on how you use your car and will take 5 to 10 minutes to complete. But it’s a problem if your journey’s too short and the regeneration doesn’t finish.
During active regeneration you may notice:
- Cooling fans running
- Faster engine idle speed
- Automatic Stop/Start doesn’t work
- Increased fuel consumption
- A hot, acrid smell from the exhaust.
- The engine sounds different
Don’t ignore a warning light
If you get a warning light showing that the filter’s blocked, it should be possible to complete an active regeneration cycle and clear the warning light by driving for 10 minutes or so at speeds over 40mph.
If you ignore a DPF warning light and keep driving in a relatively slow, stop/start pattern, soot will build up in the filter until your car goes into ‘restricted performance mode’ to prevent damage.
If you let it get this bad:
- Driving at speed alone won’t be enough.
- You’ll have to get a dealer to do a manual or ‘forced’ filter regeneration.
- In extreme cases they may have to replace the filter which can cost at least £1000 plus labour.
In most cases there’s only a short time between the DPF being partially blocked and it getting so blocked it needs a manual regeneration.
If there’s a fault with the DPF or the differential pressure sensor, which lets your car know the DPF's status, the engine management light might also come on.
If you're buying a new car and will use it mainly for town-based, stop/start driving you’d be wise to avoid diesels fitted with a particulate filter
Even if your driving isn't mainly urban/stop-start, you might need to change your driving style to keep the system working properly. Follow any advice in your vehicle handbook.
What prevents normal regeneration?
- Frequent short journeys where the engine doesn’t get hot
- The wrong type of engine oil – check your handbook
- A problem with the fuel system or Exhaust Gas Recirculation (EGR) causing excess soot.
- A warning light on the dashboard or a diagnostic trouble code stored in the engine management system.
- Going over the recommended service interval
- If the vehicle uses Eolys™ additive, a low level in the tank can prevent regeneration.
- Low fuel level – generally less than a quarter of a tank – will prevent active regeneration taking place.
Most DPFs are fitted close to the engine where the exhaust is hottest so that passive regeneration is more likely to work.
But some cars use a different type of DPF which needs a fuel additive (Eolys™ fluid) to lower the ignition temperature of the soot particles so that regeneration can occur at a lower temperature.
- Additive is stored in a separate tank and automatically mixed with the fuel.
- A full tank of additive should last around 70,000 miles.
- You’ll have to pay (around £200 for fluid and labour) to get the additive tank refilled.
- Don't ignore a warning light showing that the additive tanks need refilling – without additive the DPF will quickly become blocked.
Removal's not a legal option
It’s sometimes suggested that you can get a DPF cut out of the exhaust and the engine management software reprogrammed rather than pay to get it repaired.
- DPFs are fitted to meet European emissions regulations and it would be an offence (under the Road vehicles (Construction and Use) Regulations) to use a vehicle which has been modified in such a way that it no longer complies with the emissions standards it was designed to meet.
- Removing a DPF could also invalidate any insurance cover because it makes the vehicle illegal for road use.
- Since February 2014 a missing DPF, where one was fitted when the vehicle was built, will result in an MOT failure.
updated 7 August 2017