The particulate filter: how to tell it is struggling

Inside the housing sits a ceramic body with many parallel channels. The channels are alternately plugged: what is open at the front is closed at the back, and the other way round. The exhaust cannot pass straight through and has to cross the porous wall between two channels. The particles stay on the wall, the gas goes on.

That holds for the diesel particulate filter and for the petrol one that direct-injection petrol engines also carry. The operating conditions differ — a petrol engine’s exhaust runs hotter, which makes self-cleaning easier — but the construction is the same.

Because the filter blocks up by doing its job, it is not a component that simply sits there. It is a container that has to be emptied regularly, and the engine does that itself while driving — as long as you let it.

Soot is carbon and burns when it gets hot enough. On a longer run under load the exhaust becomes hot enough on its own for the stored soot to burn away continuously — that is passive regeneration, and it is the one that costs nothing.

When that is not enough, the control unit steps in. It estimates the load from two sources: the differential pressure across the filter, measured by a dedicated sensor, and a model built from injection quantity, engine speed and load. Above a threshold it starts an active regeneration, injecting fuel after the main combustion event so that it burns in the exhaust system and raises the temperature inside the filter.

An active regeneration takes time — minutes, not seconds — and it needs that time in one piece. You can notice it: a raised idle, a changed note, a smell, a fan that keeps running after shutdown, sometimes a stop-start system that refuses to engage.

Drive a diesel four kilometres into town and you never reach the exhaust temperature for passive regeneration, and you interrupt every active one, because the engine is switched off before it finishes. The control unit records the interruption and tries again next time — and the time after that with more effort.

That is the point where a usage pattern turns into damage. Every aborted regeneration leaves more soot behind than before, the next has to run longer, and at some stage the control unit stops trying and drops into a limp mode with reduced power. After that only a forced regeneration at a workshop, or removal of the filter, will help.

For a used purchase that means a low-mileage diesel with a city-dwelling previous owner is not automatically the better car. Asking about the usage pattern is a technical question on a diesel, not small talk.

Not everything in the filter is soot. A small part is ash: incombustible residue from the additives in the engine oil and from normal engine wear. Ash cannot be burnt away. It accumulates over the life of the filter and permanently narrows the free cross-section.

That is why even a perfectly regenerated filter is eventually full. And it is why its life depends on the oil: oils for vehicles with a particulate filter are formulated low in ash, with limited sulphated ash, phosphorus and sulphur. The wrong oil over several intervals measurably shortens the filter’s life, with nothing noticeable happening along the way.

Many control units track the estimated ash mass as a separate value. On a used car it says more than the soot figure, because it does not depend on the last journey but on the car’s whole history.

Active regeneration injects fuel that contributes nothing to combustion. Some of it passes the cylinder wall and ends up in the oil. With occasional regenerations that is immaterial; with constantly aborted ones it is not. The oil level then rises above the upper mark, and oil thinned by fuel carries less at the bearings.

On a diesel with a particulate filter, an oil level above maximum is therefore not a sign of generous topping up but, first of all, a suspicion. The dipstick is the cheapest diagnosis there is, and it belongs to every viewing — cold, on level ground, and with a check for whether the oil smells of diesel.

What a filter reports before it gives up can be observed without a workshop:

The most informative check is a diagnostic read. With access to a scan tool you see more than any test drive shows: soot loading, estimated ash mass, the distance since the last successful regeneration and the number of aborted attempts. Three of those four say something about the future rather than the moment.

The test drive still belongs in it, and it needs a stretch of fast road. A diesel that has run ten minutes under load shows whether it holds power, whether a regeneration starts and whether anything appears in the fault memory afterwards that was not there before.

And the paperwork: a filter already replaced or cleaned is not bad news, it is information. The bad case is the filter with no record at all that is twice as old as the car would lead you to expect.

If the filter is only loaded with soot, a forced regeneration at a workshop, or simply a longer run under load, is often enough. If it is loaded with ash, no regeneration helps; what remains is wet cleaning at a specialist, with the body removed and flushed, or replacement.

Removing it without replacement is not a third option. It takes out a component that is part of the type approval; the vehicle no longer matches its approved state, fails the technical inspection and is not road legal. If you are looking at a car whose history sounds like filter trouble and whose underside looks freshly worked on, ask about it.

The most honest conclusion concerns the choice of car: if most of your journeys are short, a diesel means buying technology designed for the opposite. That is not a weakness of the engine but a question of fit.