Diesel particulate filter (DPF) explained

A diesel particulate filter catches soot from the exhaust and burns it off from time to time. It works best with regular longer drives; mostly short trips can clog it.

Updated 16 October 2026

What it does

Diesel engines make fine soot particles. The particulate filter (DPF, or FAP on some French cars) sits in the exhaust and traps them. Most modern diesel cars have one.

The engine computer judges how full the filter is, mainly by the pressure difference across it, measured by a differential pressure sensor.

How it cleans itself

Trapped soot is burnt off in a regeneration. There are two kinds.

Passive

On longer drives at a steady, higher speed, the exhaust gets hot enough for soot to burn off gradually. You don’t notice it.

Active

When the filter gets fuller, the engine computer raises the exhaust temperature, for example by injecting extra fuel, to burn the soot off. It takes a while, and needs the drive to go on until it’s done.

Some cars also use an additive in the fuel to help the soot burn at a lower temperature.

Signs that an active regeneration is under way differ between cars. Common ones are a higher idle speed, the cooling fan running on after you switch off, a hot smell, and start-stop not working. If you notice them, finish the drive if you can rather than switching off.

Why short trips matter

On short trips the exhaust doesn’t get hot enough for passive regeneration, and an active regeneration is often interrupted when you switch off. The filter keeps filling up until the warning light comes on, and a badly clogged filter can put the car into limp mode.

If the car is mostly used for short trips, a longer drive at a steady speed now and then helps, if your handbook recommends it.

The warning light

Many diesels have their own particulate filter warning. What to do depends on the car: your handbook says. Often it’s a drive at a steady, higher speed for a while, so the filter can regenerate.

If the check engine light comes on as well, or the car loses power, have it checked soon. A badly clogged filter may need a regeneration started with a workshop tool, or cleaning or replacement. Read My Car can read the codes, but can’t start a regeneration.

Over many years the filter also fills with ash from the engine oil, which doesn’t burn off. Late in its life it then needs cleaning or replacing.

Related fault codes

• P2463: particulate filter clogged with soot
• P2002: particulate filter efficiency below threshold
• P2458 and P2459: regeneration takes too long or happens too often
• P0401: EGR flow insufficient
• P20EE: AdBlue (SCR) catalyst efficiency
• AdBlue (SCR) explained

EGR, injector and turbo faults can make the engine produce extra soot, so they often show up with particulate filter codes. Fix those first, or the filter fills again.

What not to do

• Don’t keep making short trips once the warning is on: the filter only gets fuller.
• Don’t switch off in the middle of an active regeneration if you can avoid it.
• Don’t clear the codes to turn the light off. Clearing doesn’t burn off any soot.
• Don’t remove or empty the filter. In the Netherlands a factory-fitted filter has to stay in place and work for as long as the registration lists it, and since 2023 the APK measures the particle count of those diesels, which a car with a removed or emptied filter fails. For some diesels first registered before 2017, the owner can have the filter data removed from the register, but the car then pays a surcharge on road tax (fijnstoftoeslag). Across the EU, roadworthiness tests count missing or clearly defective emission control equipment fitted by the manufacturer as a major defect.

Questions