DPF Warning Light On? Why Short Journeys Are Hard on Diesels
If you drive a diesel built in the last fifteen years or so, it has a diesel particulate filter, usually shortened to DPF. It is one of the most common causes of warning lights, poor running and unexpected garage bills on modern diesels, and most of the trouble comes down to how the car is used rather than any fault with it.
What the DPF does
The filter sits in the exhaust and traps the soot that a diesel engine produces. Without it, that soot would go straight out of the tailpipe. Over time the filter fills up, so the car periodically burns the trapped soot off in a process called regeneration. To do that the exhaust has to get very hot, which only happens when the engine has been working hard for a sustained period, typically a steady run at 40mph or more for fifteen to twenty minutes.
Why short journeys are the problem
If your driving is mostly short trips around town, the school run, or a few miles to work and back, the exhaust never gets hot enough for long enough to regenerate. The filter keeps filling. The car will try to help by starting an "active" regeneration, injecting extra fuel to raise the temperature, but if the journey ends before it finishes, that is abandoned too. Do that enough times and the filter blocks.
What the warning light means
The DPF light usually looks like a box with dots inside it. When it first comes on, the car is telling you the filter is getting full and needs a regeneration. At this stage the fix is simple: take the car for a proper run, on a dual carriageway or A-road, keeping the revs up, for twenty minutes or so. Do not stop and start. In most cases the light will go out.
If the light is ignored, the car will typically go into a reduced-power mode and the engine management light will join it. At that point the filter is too full to regenerate on its own, and it needs a forced regeneration at a garage using diagnostic equipment. Leave it longer still and the filter can be damaged beyond recovery, and a replacement DPF is expensive.
Things that make it worse
- Using the wrong engine oil. Diesels with a DPF need a "low SAPS" oil (the specification is in the handbook). The wrong oil leaves ash in the filter that regeneration cannot remove.
- A fault elsewhere, such as a faulty sensor, a sticking EGR valve or a leaking injector, that stops regeneration happening. This is why a DPF light should be checked rather than just cleared.
- A low fuel level. Many cars will not start a regeneration with less than a quarter of a tank.
Is removing the DPF an option?
No. It is illegal to drive a car that has had its DPF removed, it is an automatic MOT failure, and the MOT includes a check for it. We will not do it.
Is a diesel right for your driving?
If almost all of your driving is short and local, a modern diesel will always be fighting its DPF, and a petrol or hybrid may suit you better. If you do regular longer runs, a diesel is still a very good choice, and a weekly twenty-minute run at speed is usually all it takes to keep the filter healthy.
If your DPF light is on, or the car feels down on power, book a diagnostic check and we will read the fault codes, check the filter's soot loading and carry out a forced regeneration if it needs one.
Book online at bestgarages.co.uk or call your nearest branch: Newport on 01983 533360, Lymington on 01590 674776, or Fordingbridge on 01425 561504.