The diesel particulate filter is one of the most misunderstood parts of a modern diesel engine. DPF (Diesel Particulate Filter) is the general term; FAP (Filtre à Particules) is the name used by the PSA group, which put it into series production first, in 2000. Understanding how it works is the first step to diagnosing it properly and deciding whether to clean, regenerate or replace it.
What is inside the filter
Inside the metal canister there is a ceramic monolith, usually silicon carbide or cordierite, crossed by thousands of thin parallel channels. The channels are alternately plugged at one end: exhaust gas enters a channel closed at the far end and has to pass through its porous wall to leave through the neighbouring channel. Particulate stays trapped on the wall.
Often, in the same canister and upstream of the filter, there is also a diesel oxidation catalyst (DOC): it oxidises unburnt hydrocarbons and carbon monoxide and, as we will see, plays a key role in regeneration.
Efficiency is very high: a wall-flow filter typically traps more than 95% of particulate by mass. The flip side is that the filter, by design, fills up.
Passive, active and forced regeneration
The trapped particulate is mostly carbon, in other words soot, and carbon burns. Regeneration is exactly that: burning off the accumulated soot to clear the filter. It happens in three ways.
Passive regeneration
With the exhaust already hot, roughly 250 to 450 °C as on a motorway run, the nitrogen dioxide produced by the catalyst oxidises soot continuously, without any intervention from the ECU. Vehicles that cover a lot of open-road mileage have a filter that largely cleans itself.
Active regeneration
When the estimated soot load exceeds a threshold, the ECU raises the exhaust temperature to around 550–650 °C with late post-injections that burn in the catalyst. The cycle usually lasts 10–20 minutes and repeats every few hundred kilometres, depending on the engine and use. It needs steady driving to complete: if the engine is switched off halfway, regeneration is left unfinished and restarts on the next trip.
On filters with an additive, typical of the PSA group, a dedicated tank doses a cerium- or iron-based additive into the fuel, lowering the soot combustion temperature and making regeneration easier. When the additive runs out, the filter clogs much faster.
Forced regeneration
This is the one started in the workshop with a diagnostic tool: with the vehicle stationary, the ECU raises engine speed and heats the filter until combustion is complete. It recovers a soot-loaded filter, provided the cause that prevented automatic regeneration has been fixed first.
Soot and ash: the difference that matters
This is the point many overlook. The filter does not only collect soot but also ash: incombustible mineral residue that comes mainly from engine oil additives (calcium, magnesium, zinc, phosphorus) partly burnt in the combustion chamber, from the fuel additive where fitted and, to a lesser extent, from engine wear.
Ash does not burn: no regeneration, passive, active or forced, removes it. It stays in the channels, reduces the usable volume of the filter and raises its baseline back-pressure. After each regeneration the filter comes back a little less clean than before, regenerations become more frequent, and in the end the ECU reports a clogged filter even right after a successful regeneration.
| Soot | Ash | |
|---|---|---|
| Origin | Incomplete combustion of diesel | Engine oil additives, fuel additive, wear |
| Composition | Carbon | Sulphates, phosphates and metal oxides |
| Burns during regeneration? | Yes | No |
| How to remove it | Passive, active or forced regeneration | Only by cleaning the filter |
That is why a high-mileage filter can clog even on a well-used vehicle: soot is managed, ash adds up. Accumulation can be slowed with low-ash oils (low SAPS, ACEA C specifications) and oil consumption kept under control, but sooner or later the filter has to be cleaned or replaced.
What it means for the workshop
- A forced regeneration does not recover an ash-loaded filter. If back-pressure stays high after a complete regeneration, the problem is ash.
- Look for the upstream cause before the filter. Injectors, EGR, a turbo leaking oil, sensors: a clean filter fitted to an engine producing too much particulate clogs again.
- Check the oil level. Interrupted active regenerations send diesel into the oil: a rising level is a warning sign.
- Measure, do not guess. Back-pressure before and after the job is what tells you whether the filter has really been recovered.
Frequently asked questions
What is the difference between FAP and DPF?
Nothing substantial. DPF is the general term; FAP is the name introduced by the PSA group (Peugeot and Citroën), whose filters often use a fuel additive to help regeneration. The two terms are used interchangeably.
How often does a DPF regenerate?
Active regeneration starts when the estimated soot load exceeds the threshold: roughly every few hundred kilometres, more often in town driving and less on the motorway, where passive regeneration also works.
Can the ash in a DPF be burnt off?
No. It is incombustible mineral residue: the only way to remove it is to clean the filter, for example with hydrodynamic washing.