DPF problems

What a DPF does

A diesel particulate filter (DPF) traps soot from the exhaust of a diesel car. It cleans itself by burning the soot off at high temperature, called regeneration. That needs regular longer drives at steady speeds, so cars used mainly for short trips are most likely to have DPF problems.

Signs of a blocked DPF

• A DPF warning light on the dashboard.
• Loss of power, or the car going into limp mode.
• Worse fuel economy.
• The cooling fan running after you switch off, or idle speed staying high (signs a regeneration was interrupted).

What to do when the DPF light comes on

Check your handbook, as the advice varies by car. Often, a drive of 15 to 20 minutes at a steady speed on a dual carriageway or motorway lets the filter regenerate and the light go out. If the light stays on, flashes, or other warning lights join it, don’t keep driving short trips: get it checked, because a badly blocked filter is harder and more expensive to clear.

Cleaning or replacement

Garages can often clear a blocked DPF with a forced regeneration using diagnostic equipment, or by cleaning it. Replacement is usually the last resort. A good garage will also look for the cause, such as faulty sensors, an EGR fault or injector problems, so it doesn’t block again.

DPF removal is not a fix: taking the filter out is illegal on road cars and will fail the MOT.

Avoiding DPF problems

• Give the car a longer run at steady speed regularly if most of your trips are short.
• Use the engine oil specified for your car. Many DPF-equipped diesels need low-ash oil.
• Don’t ignore the DPF light or other engine warning lights.

Frequently asked questions

Can I drive with the DPF light on?

Usually yes, for a regeneration drive as your handbook describes. If the light stays on or the car loses power, get it checked.

Do petrol cars have a DPF?

Some newer petrol cars have a similar filter (a GPF), but DPF problems are far more common on diesels.

Will a DPF problem fail the MOT?

A missing DPF fails the MOT, and diesels have an exhaust smoke test that a faulty system can fail. If the engine management light is on as well, that is an MOT failure in itself.