Clean or replace: what you need to know about your DPF
Almost every modern diesel car and van has a diesel particulate filter in the exhaust. Most of the time you never think about it. Then a light comes on, the van loses power on a hill, and someone quotes you for a new one. This guide explains what the filter does, why it blocks, and how to tell whether a clean will put it right.
What a DPF does
A DPF is a block of porous ceramic, usually inside a steel canister, made up of thousands of tiny square channels. The channels are closed off at alternate ends, so exhaust gas has to pass through the channel walls to get out. The gas goes through; the soot does not. That is how a modern diesel meets the emissions standard it was built to, and why there is no black cloud when you pull away.
The drawing in the comparison panel shows a filter end-on. Each square is one channel. As you tick the symptoms your vehicle has, the drawing fills with soot to match.
How it is meant to clean itself
A filter that only collected soot would block within weeks, so it is designed to empty itself. The process is called regeneration: the soot is burned off at high temperature and leaves as gas. On a long run the exhaust gets hot enough for this to happen on its own. When it does not, the engine's computer can raise the exhaust temperature deliberately to force a burn. You may notice the fan running after you stop, a slightly higher idle, or a small rise in fuel use while it happens.
Regeneration burns soot. It cannot burn ash, the fine mineral residue left by engine oil and fuel additives. Ash builds up slowly over the life of the filter, and the only way to get it out is to clean the filter physically.
Why filters block
The most common reason is the way the vehicle is used. Manufacturers' handbooks warn that frequent short or cold journeys can stop the engine reaching the temperature the filter needs to regenerate. A van that does ten local drops a day, each a few minutes apart, may never get hot enough. Soot builds faster than it burns.
The other reasons are worth knowing, because a clean will not fix them on its own:
- an EGR valve sticking, sending more soot through the engine than it should;
- a faulty pressure or temperature sensor, so the engine never knows it needs to regenerate;
- an injector or turbo fault, putting unburned fuel or oil into the exhaust;
- a regeneration interrupted again and again, by stopping the engine halfway through.
This is why a good DPF job starts with a diagnosis. Clean a filter without fixing what filled it and it can block again within months.
The warning lights, and what to do about each
The DPF light on its own. The filter is filling and has not managed to regenerate. Check your handbook: most describe a sustained drive at steady speed that gives it the chance. If the light goes out and stays out, keep an eye on it. If it comes back within days, the filter needs more than a drive.
The light keeps coming back, or the filter seems to regenerate constantly. The filter is too full to clear itself, or something is stopping it. This is where a clean usually comes in, together with a check for the cause.
Loss of power, limp mode, or the engine management light. The engine has protected itself, or has found a fault. The handbooks say an amber DPF warning that is ignored can become a red one. Get it looked at before you drive it much further, and before anyone quotes you for a new filter: the fault may be the thing to fix first.
Visible smoke from the exhaust. On a vehicle fitted with a DPF, any visible smoke means the filter is not doing its job, or something upstream is badly wrong. It is also an MOT failure.
When a clean works, and when it does not
A clean works when the filter is blocked but sound. That covers most filters that have filled up through short journeys, failed regenerations or ash build-up. Once the soot and ash are out, the exhaust flows again and the filter goes back to work.
A clean cannot fix a filter that is physically damaged. A filter that has overheated during a regeneration can crack or melt inside, and a filter that has been knocked or corroded can break up. If that has happened, the only fix is a new filter, and Kernowsport will tell you so before they do any work.
The calculator assumes your filter is sound. If you are not sure, tick what the vehicle is doing and send the details to Kernowsport. It costs nothing to ask.
What happens at Kernowsport
Kernowsport has invested in a MetClean E MK2 DPF cleaning machine at its new site in Pool, Cornwall, set up as a centre for DPF cleaning and fault resolution for Cornish trade and retail customers. You can bring the filter in already off the vehicle, or bring the vehicle and let them remove the filter, clean it and refit it.
A typical visit looks like this, and Kernowsport will confirm the details when you book:
- Read the fault codes and check the sensors, so the cause is found as well as the blockage.
- Remove the filter, if you have booked the fitted service.
- Clean it in the machine to remove the soot and ash.
- Refit it, clear the codes and check the vehicle is working as it should.
Why removing the filter is not the answer
You may have heard that some garages will take the filter out. If your vehicle was built with a DPF, evidence that it has been tampered with is an MOT failure, and so is visible smoke of any colour from a vehicle fitted with one. A removed filter does not save money: it swaps a repair bill for a vehicle that cannot pass its test, and still needs a filter fitting to get back on the road.
Looking after the filter once it is clean
A clean resets the filter, but the habits that filled it will fill it again. A few things make a real difference, and none of them costs much.
- Give it a proper run. If most of the vehicle's work is short and local, plan a longer drive at steady speed every week or two, so the filter gets hot enough to regenerate on its own.
- Let a regeneration finish. If you notice the signs of one in progress, such as the cooling fan running on or a higher idle, keep driving for a few more minutes rather than switching off.
- Use the oil your handbook specifies. Many diesels with a DPF need a particular oil specification. The wrong oil can leave more ash in the filter, and ash is the part regeneration cannot burn.
- Act on the light early. A DPF warning dealt with in the first few days is usually a drive or a clean. Left for weeks, it can become a fault, a limp-mode recovery or a damaged filter.
- Keep the service history up to date. Sensors, EGR valves and injectors all affect how much soot reaches the filter. A vehicle that is serviced on time is a vehicle whose filter has less to do.
If the light comes back soon after a clean, that is a sign something upstream still needs attention. Tell Kernowsport when it happened and what the vehicle was doing, and they can look for the cause.
For trade and fleet vehicles
For a van that earns money, the price of the part is only half the bill. A new filter that has to be ordered in can leave the vehicle parked for days, and every one of those days is work turned away or a hire van paid for. That is why the calculator asks what a day off the road costs you. Put in your own figure and it shows the whole cost of each choice, not just the part.
If you run several vehicles on short local work, it is worth talking to Kernowsport about checking filters before the lights come on. A filter cleaned on your schedule costs a planned morning, not an unplanned week.
How to use the calculator
- Choose the type of vehicle the filter comes from.
- Choose whether Kernowsport remove and refit it, or you bring the filter in.
- Enter the price on the quote you have for a new DPF, and how long you would wait for it.
- Add what a day off the road costs you, if you know it.
- Tick what the vehicle is doing, to see whether a clean is the right next step.
The comparison updates as you type. When you are ready, the WhatsApp or email button opens your own app with your figures already written into a message to Kernowsport.
Questions people ask
Is cleaning a DPF as good as fitting a new one?
When the filter is blocked but not damaged, a professional clean removes the soot and ash that are stopping the exhaust flowing, and the filter goes back to work. A clean cannot repair a filter that is cracked, melted or broken up inside. Kernowsport check for that before they clean, and tell you if a new filter is the only fix.
How do I know if my DPF is blocked?
The usual sign is the DPF warning light on the dashboard. Others are a loss of power, the vehicle going into limp mode, the engine management light, or a regeneration that keeps starting and never seems to finish. Any of these is worth acting on early.
Will a long drive clear the DPF light?
Often, if it is caught early. Most handbooks describe a sustained drive at steady speed that lets the filter burn off its soot. Check yours for the exact drive. If the light stays on or comes back within days, the filter needs more than a drive.
Why does my DPF keep blocking?
Short or cold journeys are the most common reason: the exhaust never gets hot enough for the filter to clean itself. A fault elsewhere can also do it, such as an EGR valve, a sensor or an injector sending too much soot. That is why it pays to find the cause as well as clean the filter.
Can I just have the DPF removed?
No. If a vehicle was built with a DPF, evidence that it has been tampered with is an MOT failure, and so is visible smoke of any colour from the exhaust of a vehicle fitted with one. A removed filter also means your vehicle no longer meets the emissions standard it was built to.
How long will my vehicle be off the road?
Kernowsport will confirm the turnaround when you book. The calculator above uses their figure so you can compare it with the wait for a new filter, which depends on whether the part is in stock.
Does this work for vans, 4×4s and motorhomes?
The calculator covers cars, vans, 4×4s and pickups, and motorhomes. If you run something larger, or a fleet, message Kernowsport and they will tell you what they can take.
Can I bring the DPF in already removed?
Yes. Choose "I bring the DPF in off the vehicle" in the calculator to see the clean-only price. If you would rather Kernowsport remove and refit it, choose the fitted option.
What happens to the figures I type into the calculator?
Nothing leaves your browser. The page works out the comparison on your own device, stores nothing and sends nothing. If you press the WhatsApp or email button, your own app opens with a message you can read and change before you send it.
Why should I find the cause as well as clean the filter?
A clean puts the filter back to work, but whatever filled it will fill it again. Kernowsport handle DPF fault finding as well as cleaning, so the visit can deal with both and you are not back in a few months with the same light.
Is a cleaned DPF legal for the MOT?
Yes. A cleaned filter is the original filter, still fitted and still working. The MOT checks that the filter has not been tampered with and that a vehicle fitted with one shows no visible smoke.
Sources
- MOT inspection manual changes (new failures in section 8, nuisance), Driver and Vehicle Standards Agency, GOV.UK. Checked 4 November 2026.
- Diesel particulate filter (DPF): owner handbook pages, Jaguar Land Rover (manufacturer handbook, TOPIx). Checked 4 November 2026.
- Partnership announcement with MetClean, Kernowsport Performance Tuning, Facebook post of 4 November 2026. Checked 4 November 2026.