Most HGV DPF problems announce themselves well before the truck stops. DAF’s driver guide for its LF range, for example, sets out four escalating warning levels, and by the fourth the driver can no longer run a parked regeneration at all and the dealer has to get involved. The signs on the way there are the ones worth learning, because each one is cheaper to deal with than the next.
The warning signs, from early to urgent
| What you see | What it usually means | What to do |
|---|---|---|
| Regens coming round more often | The filter has less usable space, commonly because ash has built up | Note the frequency and plan an inspection before it turns into a warning |
| Parked (stationary) regen request | Passive and active regeneration haven’t kept soot below the set level | Run the parked regen when it is safe to, rather than putting it off |
| The same request back at every key cycle | The underlying soot level hasn’t been dealt with | Complete the regen; if it keeps returning, get the system diagnosed |
| Power reduction or derate | The engine is protecting itself and the aftertreatment | Treat as urgent and arrange diagnosis |
| Engine warning lamp on | A fault the engine management has logged, which may or may not be the filter | Read the fault codes before replacing or cleaning anything |
| Visible smoke from the exhaust | Possible filter damage, or a fault that is overwhelming it | Get it checked: it is also an annual test failure point |
Regenerations needed more and more often
This is the earliest and most overlooked sign, because nothing is technically wrong yet. The truck regenerates, the warning clears and the driver carries on. The interval between regens is the thing to watch.
Regeneration burns soot, but it can’t touch ash. DieselNet’s technical guide explains that ash is the incombustible residue left after regeneration, coming mainly from lubricating oil additives, and that it reduces the filter’s effective volume. Less room means soot reaches the trigger level sooner, so regens come round more often. On a high-mileage HGV that has never had its filter cleaned, this is very often the story.
Parked regeneration requests that won’t go away
When passive regeneration on the road and active regeneration by the engine management aren’t enough, many trucks ask the driver to run a parked, or stationary, regeneration. In DAF’s LF guide this means parking outdoors with space around the exhaust, applying the park brake, leaving the engine at idle and staying with the vehicle, and the process takes between 45 and 70 minutes depending on soot loading.
The same guide notes that if the driver doesn’t act, the request repeats at the next and subsequent key cycles. A request that keeps returning after a completed parked regen is no longer a driving-pattern problem. Something is stopping the filter clearing, and that needs diagnosing.
Here’s what we’d say most operators get wrong: they treat each parked regen as the fix. It is a reset. If the truck needs them every few days, the filter or something upstream of it needs attention.
Power reduction and derate
As soot loading climbs, the engine starts protecting itself. DAF’s guide ties its higher warning levels to engine power reduction, states that driver intervention is required immediately, and warns of further power reduction beyond that. (Its high exhaust system temperature lamp, by contrast, can appear during a normal active regen at low speed and is not a fault on its own.) Other manufacturers use different lamps and wording, but the pattern is similar: the warnings escalate and the truck gives up power to protect the engine and aftertreatment.
A derate is the point at which a DPF problem becomes an operational one. A loaded truck with reduced power is slow on gradients, can miss delivery windows and may need recovering. If you are seeing derates, stop running parked regens on a loop and get the vehicle looked at.
Rising back pressure and fuel use
Back pressure is the resistance the exhaust has to push against, and a loading filter raises it. Drivers won’t see a pressure figure on the dash, but your diagnostics will show differential pressure across the filter, and you may notice it in the fuel figures. DieselNet estimates that ash-related back pressure increases fuel consumption by about 2 to 3%.
One caution: DieselNet also notes that pressure drop alone is an unreliable measure of how much ash is in a filter. A high reading tells you there is a restriction, not what it is. A faulty pressure sensor or its pipework can also give false readings, which our guide to DPF pressure sensor faults covers in more detail.
Warning lamps, smoke and the annual test
An engine warning lamp is the sign most likely to have consequences beyond the yard. In DVSA’s HGV inspection manual, an engine malfunction indicator lamp that is inoperative or indicating a malfunction is a major defect for diesel vehicles first used from 1 July 2008. So is visible smoke from a vehicle fitted with a DPF, and so is emission control equipment that is absent, obviously defective or tampered with.
That makes an ignored DPF fault a test-day risk as well as a running problem. If a lamp is on, get the codes read well before the vehicle is due in.
Why HGVs show these problems differently from cars
The basics are the same as on a car, and our guide to what each stage of a DPF warning means explains them. What changes on a truck is the duty cycle and the mileage.
- Multi-drop and urban work keeps exhaust temperatures down, so passive regeneration rarely gets a chance.
- Long idling, whether waiting to load or running equipment, adds soot without the heat to burn it.
- High annual mileage means ash builds up far faster in calendar terms than on a private car.
- Different drivers respond differently to the same warning, so one truck in a fleet can end up far worse than its sister vehicles.
If the regens are failing rather than just frequent, our article on why DPF regeneration keeps failing walks through the usual causes.
What to do when the signs start
We clean DPFs off the vehicle at our workshop in Fenton, Stoke-on-Trent. Your workshop removes the filter and drops it off, or trade and fleet customers can post it in. Larger commercial and HGV filters are priced separately, on quote, and our new DCS-20L machine supports HGV DPF cleaning. Send us your filter details or part number for a quote, and see our off-vehicle DPF cleaning service for how it works.
Black water flushing out of a filter during a clean in our current machine.
Frequently asked questions
What are the signs of a blocked DPF on a lorry?
Watch for regenerations needed more often, parked regeneration requests that keep returning, power reduction or derate, rising fuel use and an engine warning lamp. Several of these together usually mean the filter is loading faster than it can clear.
Why does my truck keep asking for a parked regen?
Usually because soot is building up faster than the road and active regens can burn it. Short, slow or idle-heavy work is a common cause. On a high-mileage truck, ash taking up space in the filter is another, and a fault upstream can do the same. If requests keep coming back, get it diagnosed.
Will a DPF fault fail the HGV annual test?
It can. DVSA’s HGV inspection manual lists an engine warning lamp indicating a malfunction, visible smoke from a vehicle fitted with a DPF, and defective or tampered emission control equipment as major defects. Get any lamp or smoke checked well before the test.
Can a parked regeneration fix an HGV DPF?
It can clear soot, which is why trucks ask for one. It cannot remove ash, so on a filter that is ash-loaded the warnings will keep returning. At that point the filter needs to come off and be cleaned.
Seeing these signs on your fleet?
Send us the filter details or part number and we’ll quote for cleaning. Call 07827 533529 or get in touch online.
