Today's diesel cars spit out a lot less pollution than older models. A combination of a diesel particulate filter (DPF) and NOx-reducing additives, such as AdBlue, mean you can drive a modern diesel car with confidence that there'll be no clouds of black smoke pouring out of your tailpipe and that passers-by won't be left choking in your wake.
However, this emissions-reducing technology - especially the DPF - needs to be looked after if it's to continue working properly. Your vehicle's sensors and computers play a large part in the day-to-day operation of the DPF but it also requires you to alter your driving style: regularly make journeys of at least 15 minutes (when the car can get up to operating temperature) and drive on roads where you can get the car up to at least 65km/h.
But what happens when you can't drive the car as much as before? You're now limited to short trips, possibly just once a week - how does DPF regen work when you can't take the car on longer journeys?
Why a DPF gets blocked
A diesel particulate filter is designed to collect the sooty particles that are produced with the exhaust gas as part of the combustion process. Once the engine is up to temperature, this soot is converted to ash and can be safely expelled from the exhaust pipe - known as passive regeneration.
However, if the soot can't be burnt off it could build up within the filter and block the exhaust system, which is why something called active regeneration kicks in before this occurs. The car's ECU will increase the engine revs to help burn off the soot. But if this isn't allowed to complete its job you're likely to see a warning light flash up on your dashboard.
What to do if the DPF light comes on?
This is the key bit to avoid clogging the DPF. If all of your journeys are short, there's a good chance that the DPF warning light will come on from time to time. The DPF light doesn't necessarily mean that the filter is full, but that there's a danger it could reach capacity if you don't do something about it. So don't ignore it, hoping the DPF light will simply go away.
You need to go for a drive where you can get the car up to temperature and keep the revs up - preferably above 3000rpm - for a good few minutes. For example, take the scenic route to the supermarket, keeping the car in a lower gear (although there's no need to have the revs screaming).
When you get to your destination, allow the car to idle for a minute or two - listen to the engine note and look at the rev counter; are the revs higher than normal? If so, the engine may still be trying to eradicate the soot from the DPF, so don't turn the engine off until the revs return to normal. The warning light should have disappeared by now.
Sometimes, though, the DPF will continue to clog, with the warning light possibly turning red and the car going into limp-home mode, where engine revs and performance will be severely limited. The car will be almost impossible to drive and may need to be recovered. All may not be lost, though.
Can a DPF be cleaned?
Yes. This is known as forced regeneration but has to be done at a car dealer or a suitably equipped garage. Diagnostic kit will be connected and the car's ECU will be instructed to run a full active regeneration cycle.
Naturally, you'll have to pay for this work to be carried out (it's very unlikely to be covered by any warranty), as well as for a change of engine oil and filter, plus possibly a DPF cleaning solution, which will be added to the fuel tank. All of this is likely to take at least a couple of hours.
A forced regeneration usually does the trick and you can go merrily on your way. But if you're repeatedly having to take the car in for this treatment, there's a very good chance that your DPF will need to be replaced - and this is a job that can cost well in excess of $2000, if you get a garage to do it.
Fortunately, your Haynes Manual covers DPF replacement. Click here to find yours, and save yourself hundreds of pounds in garage labour bills.
Where is DPF filter located?
A DPF is found between the catalytic converter and the exhaust manifold, where the exhaust temperatures are still hot enough to burn off the soot. Your Haynes manual will take you through the replacement procedure, step by step.
Is it illegal to remove the DPF?
Deleting the DPF can land you with a fine of up to $22,000 (NSW) because it's illegal and will void your car's warranty.
Should I buy a diesel car without DPF instead?
If short journeys are now the new normal for you, and you're not tied into a finance agreement that may be expensive to get out of, then yes, it's worth considering a switch to an older diesel model that wouldn't have been fitted with a diesel particulate filter when new.
But we'd be inclined to consider other fuel types, such as petrol, hybrid or even electric - plug-in hybrids, in particular, can run on pure electric power for distances of around 50km.
It's worth noting that more and more modern petrol engines are also being fitted with particulate filters, but clogging is less of an issue because petrol exhaust particulate sizes are smaller than with diesel.