Most recent diesel cars (any manufactured after 2008) are fitted with a Diesel Particulate Filter, which traps soot from the engine before it exits into the atmosphere, and heats it to very high temperature to reduce it to very fine white ash.
These use the temperatures generated by the engine to break down the soot particles (“passive regeneration”), plus some internal electronics to monitor the system and perform further heating when required (“active regeneration”).
These DPFs can be a trap in another sense – if they break (or, as most often happens, become blocked) they can cost well over £1000 to replace. Think £1200 for a Fiat 500 and anything up to £2000 for an Audi A4.
Depending on the way the car is driven, and the type of Diesel Particulate Filter fitted, the filters can last up to 70,000 miles before needing replacement, but doing lots of short journeys in a diesel car increases the chance of them becoming blocked. If the DPF warning lights starts flashing up on the dash, the advice is to hit the motorway for a good 15-minute run (actually any speed over about 40mph will do; try to keep revs up around 2000-2500) to trigger the passive regeneration and clear out the filter.
Some Diesel Particulate Filters need a liquid additive to work properly, which is stored in a separate tank and also needs to be refilled during a service, but again this should be very infrequent if the car gets plenty of mixed mileage and not just school runs.
No DPF = No MOT
Another problem to be wary of with DPFs is when buying a used diesel car, van or light commercial vehicle. In the past some people have taken to removing the DPFs to avoid blockages and because it was thought to improve the fuel economy.
BUT From February 2014 any vehicle which was manufactured with a DPF, but is found to no longer have one fitted, will fail its MOT.
Despite this there are still companies advertising online to remove your DPF and remap the engine for “better performance and fuel economy”… don’t do it folks!
If you are getting a DPF warning light on your dash and the “15-minute run” exercise doesn’t clear it then (a) it won’t clear by itself and (b) it could catch fire and do even more expensive damage – come and see us instead and have it looked at; it might be possible to clean it rather than replacing it.