Servicing6 min read
DPF warning light: what it means and how to clear it
A DPF light is the car asking for a long, steady drive. If that does not clear it, something is stopping the filter from cleaning itself — and that is what needs fixing, not the filter.
By Kaine Carter
Every modern diesel has a particulate filter in the exhaust. It traps soot so it does not come out of the tailpipe, and every so often it burns that soot off by getting very hot — a process called regeneration. The warning light means regeneration is not keeping up.
Why a DPF blocks
Nine times out of ten it is the kind of driving the car does. Regeneration needs the exhaust hot for a sustained period, and a car that only does short trips — school runs, a few miles to work — never gets there. The soot builds up and the light comes on.
The rest of the time, something mechanical is either producing too much soot or stopping regeneration from happening:
- A leaking or worn injector putting too much fuel in.
- A sticking EGR valve sending sooty exhaust gas back through the engine.
- A failed glow plug or temperature sensor.
- A differential pressure sensor misreading how full the filter is.
- Low or wrong-specification engine oil, which adds ash the filter cannot burn.
What to do when the light comes on
Check your handbook first, because manufacturers differ. For most cars, the advice is a steady run on a dual carriageway or motorway, in a gear that keeps the revs up, for around 20 minutes. If the light goes out, the car has regenerated and you can carry on as normal — though if it keeps happening, your driving pattern is the cause.
If the light stays on, flashes, or the car loses power and goes into limp mode, stop trying to clear it with driving. A filter that is too full can no longer regenerate on its own, and pressing on can make the job bigger.
Why removing the filter is not the answer
You will find people offering to take the filter out and code the warning away. On a road car that is illegal, it is an automatic MOT failure, and it gives your insurer grounds to refuse a claim. We do not do it, and we would be wary of anyone who offers to.
How we clean a DPF properly
- 1.Diagnose first: read the soot and ash loading, and test the pressure sensor against what the exhaust is actually doing.
- 2.Check the parts that make regeneration possible — injectors, EGR, glow plugs and sensors — so the real cause is found.
- 3.Clean the filter on the car: a forced regeneration where the loading allows it, or a chemical clean through the sensor port where it does not.
- 4.Road test to confirm the regeneration completes.
The filter stays fitted, so the car still passes its MOT. DPF cleaning and diagnosis starts from £249, done at your address in Milton Keynes, Leighton Buzzard, Bedford, Buckingham and Northampton. If a filter is too full of ash to clean — soot burns off, ash does not — we tell you and price a replacement before doing anything.
