A diesel particulate filter (DPF) traps the soot particles in diesel exhaust before they reach the air. Unlike a catalytic converter, which gases flow straight through, a DPF is a genuine filter: its honeycomb channels are blocked at alternate ends, so exhaust gas is forced through the porous channel walls. The gas passes; the soot is too large and stays trapped inside.
DPFs became effectively universal on diesel cars with the Euro 5 emissions standard — in practice, almost every diesel registered from about 2009 onwards has one, and many earlier diesels do too. Petrol equivalents (GPFs — gasoline particulate filters) are now appearing on newer petrol cars for the same reason.
Regeneration: how a DPF empties itself
A filter that only ever collected soot would block within weeks. Instead the vehicle periodically burns the trapped soot off at high temperature, converting it to a small amount of ash — a process called regeneration. On a healthy vehicle this happens roughly every 300 miles, though it varies a great deal with the vehicle and how it’s driven. There are three kinds:
Passive regeneration
On a sustained run — motorway driving being the classic case — the exhaust gets hot enough for the soot to burn off by itself. No intervention, and you’ll never notice it happening.
Active regeneration
Most cars don’t get enough sustained running, so when the filter reaches roughly 45% full, the engine control unit (ECU) steps in: it adjusts fuel injection to raise the exhaust temperature and burn the soot off deliberately. If your journeys are short and stop-start, an active regeneration may keep getting interrupted before it completes — this is how DPFs on town-driven cars gradually clog. When that happens, the DPF warning light comes on to ask for your help: a sustained drive — typically 10 to 25 minutes at 40mph or more, following the advice in your handbook — usually lets the regeneration finish and the light go out.
Forced regeneration
If the warnings are ignored and soot loading climbs to around 75%, driving alone will no longer trigger a regeneration — a garage must connect diagnostic equipment and command one. Beyond roughly 95% full, the ECU will typically put the vehicle into limp mode to protect the engine, and at that point the filter usually cannot be recovered and must be replaced.
Additive systems
Some vehicles (many Peugeot, Citroën and Ford models among them) use a fuel additive, dosed automatically from a tank near the fuel tank, to help soot burn at lower temperatures. If the additive runs out, regeneration quietly stops working — one of several reasons a DPF can block through no fault of its own. Our guide to why DPFs block covers the full list.
If your DPF has failed
A blocked DPF is almost always a symptom of something else — a vehicle fault or a driving pattern that prevented regeneration. Diagnose and fix the cause first, then replace the filter; otherwise the new one will block too, often within a few hundred miles. Find the right replacement with our part finder or browse the DPF range — and note that many of our DPFs are available in a premium silicon-carbide version with a longer warranty (the “P” suffix — see understanding our part numbers).