DPF Warning Light On? What It Means and What to Do

The DPF light is one of the most common warnings on a modern diesel, and one of the most misunderstood. Most of the time it’s the car asking for help clearing soot, not a sign of disaster. Ignore it, though, and a simple regeneration can turn into a blocked filter and a much bigger bill. Here’s what the light means, what to try first, and when to stop and get it looked at.

What the DPF does

The diesel particulate filter sits in the exhaust and traps soot so it doesn’t come out of the tailpipe. Every so often the car burns that soot off, a process called regeneration.

• Passive regeneration happens on its own when the exhaust is hot enough, such as on a steady motorway run.
• Active regeneration is when the engine’s ECU deliberately raises exhaust temperature, usually by injecting extra fuel, once the filter reaches a set soot level.

If a regeneration can’t finish, soot keeps building up and the warning light comes on.

Why the DPF light comes on

• Short journeys. The exhaust never gets hot enough, or regenerations keep getting cut short. This is the most common cause on diesels used for school runs and short commutes.
• Switching off mid-regeneration. Signs a regen is under way include the cooling fan running after you switch off, a slightly higher idle and a hot smell.
• Faulty sensors. A failed differential pressure sensor or a split sensor pipe can trigger the light even when the filter itself is fine.
• An engine fault making extra soot. A sticking EGR valve, a leaking injector or a boost leak all increase soot. Clean the DPF without fixing the cause and it will block again.
• The wrong oil. Many DPF-equipped diesels need low-ash (“low SAPS”) oil. The wrong oil leaves ash that regeneration can’t burn off.
• Low additive. Some models use a fluid additive to help regeneration, and show a warning when it runs low.

What to do when the light comes on

1. Check your handbook. Many recommend a steady drive of around 15 to 20 minutes at a constant speed above about 40mph, in a gear that keeps the revs up, so the car can regenerate. Follow what yours says.
2. If the light goes out, the regeneration worked. If it keeps coming back, something is causing it, so get it checked.
3. If the light stays on, other warning lights appear, or the car goes into limp mode, stop trying. Repeated attempts on a heavily blocked filter can make things worse. Book a diagnostic instead.

What you shouldn’t do is keep driving on it for weeks and hope it clears. A filter that’s left to block completely is much harder to recover.

How we deal with DPF problems

DPF work is done in-house at our workshop near Bedford. Depending on what we find, that means:

• Diagnostics: reading fault codes and live data such as soot load, differential pressure and exhaust temperatures, so we know whether the problem is the filter, a sensor or something upstream.
• Forced regeneration: using diagnostic equipment to run a controlled regeneration in the workshop, where the filter is suitable for it.
• DPF cleaning: for filters that regeneration alone can’t clear.
• Sensor and pipe faults: replacing failed pressure sensors and split pipes that cause false warnings.

If something upstream is making too much soot, we’ll talk you through it before the filter is cleaned, so you’re not paying to clear it twice. Every car is different, so we’ll give you a price once we’ve heard the symptoms. DPF cleaning and regeneration.

Why the fix is repair, not removal

Removing the DPF from a car used on the road is illegal. It will fail the MOT and can invalidate your insurance. If your car is used on the road, the answer is to get the DPF working properly again, which is exactly what we do.

Short trips around St Neots, Huntingdon and Bedford

If most of your driving is short runs around St Neots, Huntingdon or Bedford, your diesel may rarely get hot enough to regenerate on its own. A regular longer run on the A1, A14 or A421 helps. Our workshop at Little Staughton is about 15 minutes from St Neots and around 25 minutes from Huntingdon and Bedford, and under an hour from Milton Keynes and Luton.

Frequently asked questions

DPF light on? Talk to us

Tell us the car, the warning lights and how you drive it, and we’ll tell you what we think is going on. Call or WhatsApp 07460 752897, Monday to Friday 10am to 6pm and Saturday 10am to 4pm.