What is a DPF and how to look after it

DPF stands for diesel particulate filter. It sits in the exhaust and traps the fine soot a diesel engine produces so it doesn't go out of the tailpipe and into the air. Almost every diesel car built from around 2009 onwards has one, and it's there because of emissions rules, not because the manufacturer chose to fit it.

This is a diesel-only part. If you drive a petrol car you don't have a DPF to worry about, so this guide is for the diesel drivers.

The filter can't just fill with soot forever, so it burns that soot off at high temperature and turns it into a tiny amount of ash. That burn-off is called regeneration, or regen for short, and it happens in two ways.

Passive regen happens on its own when you drive far enough and fast enough for the exhaust to get properly hot, think a decent run at motorway or dual-carriageway speed. Active regen is when the car decides the filter is getting full and triggers a burn itself, usually by injecting a little extra fuel to raise the exhaust temperature. You often won't notice active regen, though you might see the revs sit a bit higher, the engine sound change slightly, or a hot smell after you park.

The problem is that regen needs heat, and lots of short, cold, stop-start journeys never let the exhaust get hot enough for long enough. If most of your driving is a few minutes to the school, the shops and back, the filter keeps collecting soot but rarely gets the chance to burn it off.

It's even worse if an active regen gets interrupted. If the car starts a burn and you switch the engine off partway through, that regen doesn't finish, and the soot keeps building. Do that enough times and the filter gradually blocks.

When soot builds up past a certain point, a DPF warning light comes on (usually an orange symbol that looks like a box with dots inside it). That light is telling you the filter is getting full and the car wants to run a regen it hasn't been able to complete.

If you catch it early, you can often clear it yourself. Take the car for a steady drive of fifteen to twenty minutes at a sustained higher speed, somewhere like a dual carriageway, and hold a constant speed to give the car a chance to finish a regen. If the light goes out, you've sorted it. If it stays on, or other lights join it, don't keep driving on and hoping, get it looked at before it turns into a blockage.

Most DPF problems come down to how the car is driven and whether it's been serviced properly. A few simple habits keep the vast majority of them out of trouble.

Give the car a proper longer run at speed regularly so it can passively regen. Make sure it's serviced on time using the correct low-ash oil your handbook specifies, because the wrong oil produces more ash and clogs the filter faster. And if a diesel doing nothing but short hops is your normal, it may be honest to ask whether a diesel is the right car for that driving at all.