Clogged DPF: Symptoms, Regeneration and Costs

What a diesel particulate filter is

The DPF (called an FAP in PSA-group and Ford cars) is a filter in a diesel's exhaust system that traps soot before it reaches the atmosphere. Soot builds up in it gradually, so every so often the filter has to clean itself, i.e. burn off the accumulated deposit at a high temperature. We call this process regeneration.

The trouble starts when regeneration can't run to completion. Soot keeps building up, the filter clogs and the car drops into reduced-power (limp) mode.

Why the DPF clogs, especially in the city

Warsaw is tough on a diesel with a DPF. The most common causes of clogging are:

• Short trips: the engine and exhaust never reach the temperature needed for regeneration (around 550–600°C).
• Stop-and-go traffic: regeneration starts but gets interrupted before it can finish.
• Frequent cold starts and short-distance driving.
• The wrong oil: a DPF needs low-SAPS oil (ACEA C2 or C3 spec). Ordinary oil chokes the filter with ash.
• Other faults: a leaking injector, a worn turbo or a carboned-up EGR add soot faster than the filter can burn it off.

A car used mainly for short hops around town is a textbook case of DPF clogging.

Symptoms of a blocked DPF

• the DPF warning light on the dashboard (a filter symbol),
• power loss and a switch to limp mode,
• higher fuel consumption,
• more frequent regeneration attempts (consumption briefly rises, the fan runs after you switch the engine off, you can smell hot exhaust gases),
• raised idle speed,
• a rising oil level and a smell of fuel in the oil.

That last symptom is especially dangerous. During failed regenerations, fuel drains into the sump and dilutes the oil. Thinned oil lubricates poorly, which threatens not just the filter but the turbo and the whole engine.

Types of regeneration

Passive

This happens on its own during longer runs at higher speed, for example on the S8 or a ring road. The exhaust gases are hot enough by then to burn off the soot without any input from the control unit.

Active

The control unit injects extra fuel itself to raise the temperature and burn off the deposit. It needs a good ten to fifteen minutes of continuous driving. Switch the engine off during that time and the process is interrupted.

Forced (service)

When the filter is too clogged for self-regeneration, we run it with a computer on the bench, controlling the temperature and the differential pressure across the filter. It's a safe method for a heavily sooted DPF.

When driving alone is no longer enough

Once the fill level crosses the safety threshold, the control unit blocks self-regeneration to prevent an uncontrolled burn-off. That's when a forced regeneration is needed. If even that doesn't help, the filter has to be taken out and cleaned on a dedicated flushing machine, and if it's permanently damaged, replaced.

How much it costs

The gap between reacting early and a full replacement is huge, so there's no point putting it off once the warning light comes on.

How we check the state of a DPF

Before we decide whether the filter can still be saved, we read a few specific parameters from the control unit:

• the amount of soot collected in the filter (given in grams),
• the differential pressure across the filter at various engine speeds,
• the mileage and time since the last successful regeneration,
• the number of interrupted regenerations.

This data tells us more than the warning light alone. A filter whose soot level is within spec can usually just be regenerated. With very high differential pressure and a large amount of ash (rather than just soot), cleaning on a flushing machine is sometimes necessary, because ash can no longer be burned off.

Why removing the DPF isn't worth it

Cutting the filter out and switching it off in the software (a so-called DPF delete) is illegal in Poland. It means failing the roadworthiness test, higher emissions and a real risk of a fine and having your registration document confiscated. A working DPF is now a condition for a car to be road-legal, and its absence also lowers the car's resale value.

How to look after a DPF

• Every now and then, treat the car to a longer run, 20–30 minutes at higher revs.
• Use the right low-SAPS oil and change it every 7,000–10,000 km.
• Don't switch the engine off during an active regeneration if the car signals one.
• Deal with other faults (EGR, injectors, turbo) that speed up clogging.

Twin Torque Service in Targówek

At our garage in Warsaw at ul. Chodecka 1D, we start with diagnostics and a differential pressure measurement to judge whether the DPF can still be saved by regeneration or whether it needs cleaning on a flushing machine. We always check the cause of the clogging too, because replacing the filter without removing the source of the soot is money thrown away.