DPF problems: what to know before buying a diesel

Most modern diesels have a diesel particulate filter (DPF) in the exhaust. It traps soot and burns it off from time to time, but that only works if the car gets regular runs at a steady speed. Drive a diesel mostly on short trips and the filter can block, light a warning on the dashboard and lead to a costly repair. So the first question when buying a used diesel is not about the car. It is about you: does your driving suit a DPF? If it does, check the car's history for DPF work and make sure the filter is still fitted.

What a DPF does

A diesel particulate filter is a device in the exhaust that filters soot, known as particulate matter, out of the exhaust gas. Government guidance from DVSA and the Department for Transport describes it as trapping solid particles while letting the gases pass through. Particulate matter is harmful to health, which is why the filter matters and why the law protects it.

The filter does not empty itself by magic. As it fills with soot, it has to be cleared by a process called regeneration.

How regeneration works and why short trips block the filter

The same guidance note explains that regeneration burns the trapped soot to gas at a very high temperature, leaving only a small residue. There are two broad ways this happens:

• Passive regeneration. On a longer run at a steady speed, the exhaust gets hot enough to burn soot off on its own.
• Active regeneration. When the filter is filling and the car is not getting those runs, the engine's computer raises the exhaust temperature itself, for example by changing how fuel is injected. This takes a while and needs the car to keep going.

Short trips are the problem. The exhaust never gets hot enough for passive regeneration, and an active regeneration may be cut short every time you switch off. The soot builds up. The guidance note warns that regeneration that is not carried out properly can lead to a build-up of soot, poorer performance and, in the end, expensive repairs.

Some systems also use an additive, kept in a separate tank, that helps the soot burn at a lower temperature. Where a car has one, it needs topping up as part of servicing. The handbook will say.

The DPF warning light

Most cars with a DPF have a warning light for it. The exact symbol and colour vary, so check the handbook. In general, the light means the filter is filling up and the car needs a regeneration.

The usual advice in handbooks is to drive at a steady speed for a period, on a suitable road and within the speed limit, to let the system regenerate. If the light stays on, or other warnings join it, the car needs a garage.

Signs a DPF may be struggling include:

• the DPF light coming on often;
• the engine going into a reduced-power mode;
• the cooling fan running on after you switch off, which can mean a regeneration was interrupted;
• the engine management light on as well.

Our guide to dashboard warning lights explains how to check that all the lights work at a viewing.

Forced regeneration, cleaning and replacement

If driving does not clear the filter, a garage has options. In rough order from least to most work:

1. Forced regeneration. A garage uses diagnostic equipment to tell the engine to run a regeneration while the car is parked or on a run.
2. Cleaning. The filter is cleaned, either on the car with a cleaning product or removed and cleaned off the car.
3. Replacement. A new or remanufactured filter is fitted.

A blocked DPF is often a symptom. A fault elsewhere, such as a faulty sensor, injector or exhaust gas recirculation valve, can make the filter clog. A good garage looks for the cause, not just the filter. Ask for the invoices for any DPF work and read what was found.

We do not give repair prices here because they vary widely with the car and the garage. Get a written quote for the specific car.

Removing a DPF is illegal and fails the MOT

Some firms offer to remove the filter. Do not buy a car where this has been done.

• It is an offence to use the car. The guidance note says it is an offence under regulation 61A(3) of the Road Vehicles (Construction and Use) Regulations to use a vehicle modified so it no longer meets the emissions standards it was designed to meet. Removing a DPF will almost always break that rule. The note gives fines of up to £1,000 for a car at the time it was written.
• It fails the MOT. Since February 2014 the MOT has included a check that a DPF is present where one was fitted when the car was built. The DVSA inspection manual makes "evidence that the diesel particulate filter has been tampered with" a major defect.
• Cut and rewelded filters are rejected. The manual tells testers to reject a DPF that has clearly been cut open and rewelded, unless there is evidence of a valid reason, such as cleaning. If a filter has been cleaned that way, keep the invoice.

A car can have its filter "hollowed out" so it looks present from outside. A tester may still spot signs of tampering, and the smoke test may catch it. If a seller says the DPF has been "deleted", "removed" or "mapped out", walk away.

Visible smoke from a DPF diesel

A working DPF should leave a diesel's exhaust with no visible smoke. The DVSA explained when the MOT changed on 20 May 2018 that a car with a DPF gets a major fault if the tester can see smoke of any colour from the exhaust. The inspection manual lists "exhaust on a vehicle fitted with a diesel particulate filter emits visible smoke of any colour" as a major defect.

There is also a smoke meter test. For diesel cars first used on or after 1 January 2014 the manual sets a default limit of 0.7m-1, unless the manufacturer's plate on the car gives a different figure. Our guide to the MOT emissions test explains the limits in more detail.

At a viewing, ask to see the car started from cold and watch the exhaust. Then watch it again when you rev it gently once warm. Visible smoke on a DPF diesel is a reason to stop and ask questions.

Should you buy a diesel with a DPF?

Match the car to your driving.

• It suits you if you regularly drive at a steady speed for a decent stretch, such as motorway or A-road commuting.
• It may not suit you if most of your driving is short trips in town, school runs or journeys of a few miles.

If your driving is mostly short trips, a petrol, hybrid or electric car may be a calmer choice. That is not a rule, just a fit question.

When you do buy a DPF diesel:

• read the MOT history for emissions fails and advisories, and match them to invoices (our guide to reading MOT history helps);
• ask whether the DPF has been cleaned or replaced, and see the invoice;
• ask the seller how the car has been driven;
• check the DPF and engine management lights come on at ignition and go out;
• see the car and ask a mechanic if anything feels off.

Frequently asked questions

What causes a DPF to block?

Mostly short trips. The filter needs the exhaust to get hot for long enough to burn off soot. If it never does, or a regeneration keeps being interrupted, soot builds up. Faults elsewhere in the engine can also make it clog.

Is it illegal to remove a DPF?

Using a car on the road after its DPF has been removed will almost always be an offence, according to DVSA and DfT guidance, and it fails the MOT.

Will a car with a DPF fail the MOT for smoke?

It will get a major fault if the tester can see smoke of any colour from the exhaust. It must also pass the smoke meter test for its age.

Can a blocked DPF be cleaned?

Often, yes. A garage may try a forced regeneration first, then cleaning, and replacement only if those do not work. Ask what caused the blockage, so it does not come straight back.