DPF problems: warning light, blocked filter and how to fix it

How a DPF works

The filter fills up with soot as you drive. It empties itself by getting hot enough to burn the soot to ash, called regeneration. On a long, steady run at motorway speed this happens by itself (passive regeneration). When the filter gets fuller, the engine's computer (ECU) raises the exhaust temperature on purpose by changing how fuel is injected (active regeneration), usually for 10 to 20 minutes.

Some cars, mostly Peugeot, Citroën and some Ford diesels, also use an additive from a separate tank (often called Eolys fluid) to help the soot burn. When that tank runs low, the DPF blocks up quickly.

Signs your DPF is blocking

• The DPF warning light on the dashboard (usually an orange box with dots inside).
• Less power, or the car going into limp mode with the engine management light on.
• The cooling fan running, a hot smell, idle speed higher than usual or the stop-start not working after a short drive: signs a regeneration was in progress.
• Fuel economy getting worse, or the oil level rising (diesel getting into the oil from repeated regenerations).

Why DPFs block

The most common cause is lots of short, slow journeys: the exhaust never gets hot enough, and active regenerations keep being interrupted when the engine is switched off. Other causes are a fault that makes extra soot, such as a sticking EGR valve, a leaking injector, a failing turbo or a split intake hose; a faulty DPF pressure sensor or its pipes giving false readings; the additive tank running empty; or the wrong engine oil. DPF cars need a low-ash (low SAPS) oil, and the wrong one can block the filter with ash over time.

What to do, cheapest first

• Drive it: if the light has just come on, 20 minutes or so at a steady 50 to 70 mph on a dual carriageway or motorway, in a lower gear to keep the revs up, often clears it. Check your handbook for what your car needs.
• Get the fault codes read: a scan tells you how full the filter is and whether a sensor, the EGR valve or something else is the real cause. Fix that first or the DPF will block again.
• Forced regeneration: a garage or specialist can make the car regenerate while it's parked, with a diagnostic tool.
• Professional cleaning: if regeneration won't work, many specialists can take the DPF off and clean it, which often costs much less than a new filter.
• Replacement: only when the filter is cracked, melted or too full of ash to clean.

Buying a used DPF

A used DPF can save a lot, but you can't see inside it, so ask how many miles it has done, why the car was broken, and whether it's been cleaned or tested. A DPF from a low-mileage car that was written off after an accident is a much better bet than one from a high-mileage car that may itself have been blocking. Match the part number exactly (where to find it): on many diesels the DPF and the catalytic converter are one unit, and the pressure sensor pipes and mountings differ between versions. Some cars need the new filter registered with a diagnostic tool after fitting. See also buying a used catalytic converter.

Removing the DPF is not a fix

It's illegal to drive a car on the road with its DPF taken out, it will fail its MOT if the tester can see the filter has been removed, and it can affect your insurance. A diesel with a DPF also fails the MOT if it gives out visible smoke of any colour. Repair or replace like for like.

Find a DPF or the part behind the problem

Search by your registration for DPFs, EGR valves, pressure sensors, injectors and turbos listed as fitting your car, from UK breakers and parts sellers. Search for a part, browse by make, or read how to find the right part.