DPFs Explained: Why Your Diesel’s Particulate Filter Blocks

If you drive a diesel from the last fifteen years or so, it almost certainly has a DPF. Most of the time you’ll never know it’s there. Then one day a warning light comes on, and it’s worth knowing what it means before it turns into a big bill.

What a DPF does

The diesel particulate filter sits in the exhaust and catches the soot a diesel engine makes, so it doesn’t come out of the tailpipe. As it fills up, the car burns the soot off by running the filter very hot. That’s called regeneration.

On a good run it happens on its own and you won’t notice. The trouble starts when it doesn’t get the chance.

Why they block

Regeneration needs the exhaust to get hot and stay hot, and that takes a decent run at a steady speed. A diesel that only ever does short trips never gets there, so the soot keeps building up.

Other things make it worse: a faulty sensor, an injector or EGR fault making extra soot, or the wrong oil. DPF cars need a low-ash oil for a reason.

The warning light

When the filter’s getting full, a DPF light comes on. On most cars it’s asking for a run of 15 to 20 minutes at a steady speed so it can regenerate. Your handbook will tell you exactly what your car needs.

If the light stays on after that, or other warning lights join it, get it looked at. Keep driving it like that and the filter can block completely, and then it’s a much bigger job.

How to avoid problems

• If you mostly do short trips, give it a proper run at a steady speed regularly
• If the fan’s running and the idle is higher than usual when you stop, it may be mid-regeneration, so keep going a few more minutes if you can
• Use the oil your car’s manufacturer specifies
• Get underlying faults fixed, as a bad sensor or injector will keep blocking the filter

If almost all your driving is short trips, it’s fair to ask whether a diesel is the right car for you at all.

Removal is not the answer

You’ll see adverts offering to remove the DPF. Don’t. It’s illegal to drive a car with its DPF removed, and it’ll fail the MOT.

How we do it at GT

We find out why the filter’s blocking first, because a regeneration on its own won’t last if a fault is causing it. A forced regeneration is £120 and a DPF flush and clean is £300. We’ll never remove one.

See our DPF service →