Diesel Particulate Filters

Modern diesel cars need to meet world wide emissions regulations and as such, manufacturers are now fitting Diesel Particulate Filters to the exhaust system to minimise the emissions from the car.

If you own one of the newer diesel engine vehicles, you may get a DPF warning light on your dash. A typical warning light is displayed below but will vary according to the manufacturer.

DPF filter warning light


A Diesel Particulate Filter (DPF) is effectively a soot trap in the exhaust system which filters out the black smoke you sometimes see on older diesel cars, particularly when they are under heavy acceleration. The exhaust gasses pass through the filter, leaving the particulate matter on one side and then carry on through the exhaust and out of the tailpipe. As you might imagine, having such a filter in the exhaust system is very restrictive to gas flow and as a result, limits the car in terms of power. The extra restriction on the exhaust system will also mean that the engine and particularly the turbo have to work harder which means reduced fuel efficiency compared to the same vehicle without a DPF. The reduced fuel efficiency becomes more apparent as the filter gets filled up and therefore more restrictive.

In order for a DPF to efficiently burn off the particle matter inside of it, the engine needs to be nice and hot. Many diesel car owners are not aware of this and it is because of this that some owners are having problems earlier than they might expect (assuming that they are even aware they have a DPF on the car in the first place). Clearly, to get a car hot, it needs to be driven. The first 10 minutes of any journey from a cold start are when the car produces the most emissions and so, if the car is being driven for short times only, to the shops and back, to drop the children off at school or even on just some slow town driving, the engine will not get hot enough to burn off the particle matter in the filter. It is this kind of driving that produce, more problems with DPFs than any other.

The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF
If you do a lot of short journeys, try, at least once per week to get the car out on the open road and get it to full operating temperature. That means a drive of 30 minutes or so on a good A-Road or dual carriageway. This will ensure the filter gets nice and hot and has the best chance to burn off any soot efficiently. It’s not going to eliminate the problem completely but will certainly extend the life of your DPF, the cost of which by the main dealer (some cars can only have this part replaced by main dealers) can run into a couple of thousand pounds!

If you do find that your car is feeling far more sluggish than normal and/or there is a DPF warning light on the dash – DO NOT IGNORE IT. Driving the vehicle with a blocked DPF can seriously damage your engine / turbo and it’s not uncommon to have to replace a turbo when the warning light has been ignored for extended periods.


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