What Is a DPF (Diesel Particulate Filter)?

The DPF warning symbol, usually a box with dots inside it, means the filter has collected more soot than it should and the car wants to clear it. Take the car for a steady twenty minute run at around 2,000rpm on a dual carriageway and it will often regenerate on its own. A light that stays on afterwards, or turns red, needs the workshop. Read our DPF warning light guide or ring 01744 808586.

Regeneration is the process of burning trapped soot into a small quantity of ash. Passive regeneration happens by itself on longer runs when exhaust temperatures are naturally high enough. Active regeneration is triggered by the engine management, which injects extra fuel to raise the exhaust temperature deliberately, and takes ten to twenty minutes of continuous driving. Interrupting an active regeneration repeatedly is what causes most blockages. Our diagnostics team can read soot load and regeneration history from the engine data.

Short journeys are the main reason. The exhaust never reaches the temperature a regeneration needs, so soot accumulates faster than it clears, and each aborted attempt adds unburnt fuel to the engine oil. Faults compound it: a leaking injector, a sticking EGR valve, a failed differential pressure sensor or low fuel in the tank can all block regeneration. A diesel doing only town miles is fighting its own design. Book a diagnostic session at our St Helens garage.

Cleaning works well in most cases and is far cheaper than a new filter. A forced regeneration through the diagnostic tool clears lighter blockages, and chemical cleaning through the sensor port handles more stubborn ones. Heavily loaded filters come off the car and go through a specialist cleaning machine that flushes them in both directions. We assess soot and ash levels first to pick the right method. See our DPF cleaning page for what determines the approach.

Removing a particulate filter makes the vehicle illegal to use on the road, and since 2014 the MOT includes a visual check for its presence. A missing filter is an automatic failure, and using the car on the road afterwards can bring a fine. Insurance can also be affected because the vehicle no longer matches its type approval. Cleaning or replacing is the proper route. Our DPF advice page explains the rules, and we book MOTs in St Helens daily.

Look for the DPF light, a loss of power or limp mode, a strong smell of hot fuel after driving, an idle that sounds different, and rising fuel consumption. Many cars also show the engine oil level creeping above the maximum mark, because failed regenerations dilute the oil with diesel. Some drivers notice the cooling fans running hard after parking. Any of these deserves attention. Book a check through our contact page or call 01744 808586.

Diesel cars face a strict smoke opacity test, and the MOT also includes a visual inspection to confirm the filter is present and has not been cut open and refitted. Any evidence of tampering fails the test. A partly blocked filter often still passes but leaves the car close to the limit. Getting it cleaned beforehand is the safer route. See our list of common MOT failures and book a pre-MOT check.

Diesels with a particulate filter need a low SAPS oil, meaning low sulphated ash, phosphorus and sulphur, usually described as C1, C2, C3 or C4 under the ACEA classification. Standard oil leaves far more ash in the filter, and ash cannot be burned off during regeneration, so the filter fills permanently. Using the wrong oil shortens filter life dramatically. We use the correct grade on every oil and filter change and record it on the invoice.

A well maintained filter often reaches 100,000 to 150,000 miles before ash loading forces a replacement. Cars doing regular longer journeys with the correct oil frequently exceed that. Cars used only for short urban trips can block one within a few years. Ash accumulation is the true limit, since soot can always be burned away but ash cannot. Regular cleaning extends the life considerably. Talk to our technicians about a maintenance plan, and see the cleaning options.

They are two separate systems that work together on modern diesels. The particulate filter traps soot; the AdBlue system injects urea into the exhaust to convert nitrogen oxides into nitrogen and water across a selective catalytic reduction catalyst. A car can have both, and a fault in the AdBlue system will often stop the car restarting once a countdown expires. Both need looking after. Our DPF and AdBlue page explains how they interact.

A petrol or hybrid suits short urban driving far better, because a diesel needs regular longer runs to keep the particulate filter clear. Where you already own a diesel and your driving has changed, building in one longer journey a fortnight makes a real difference. Watch the oil level too, since dilution from failed regenerations raises it. Our car buying advice covers the choice, and our servicing team can advise on your usage.

Fuel additives help in mild cases by lowering the temperature at which soot burns, which makes a passive regeneration more likely on shorter journeys. They are a maintenance aid rather than a cure for a filter that is already heavily blocked. Some manufacturers, notably PSA models, use a factory additive stored in a separate tank that needs topping up periodically. We can check whether yours has one. Ring 01744 808586 or book a carbon clean for a heavily sooted engine.

A sticking EGR valve floods the inlet with soot, a worn turbo seal pushes oil into the exhaust, a faulty differential pressure sensor gives the engine management false readings, and a leaking injector puts too much fuel in. Any of these will block a filter regardless of how the car is driven, which is why replacing a filter without diagnosis often ends badly. See our EGR valve page or book injector cleaning with us.