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

What the DPF light means

The DPF warning light means the diesel particulate filter in your exhaust is filling up with soot faster than the car can burn it off. In many cases a steady run at a constant speed, such as 20 minutes or so on a dual carriageway or motorway, will let the filter clean itself. If the light stays on after that, or other warning lights join it, get the car checked soon.

The exact advice varies between makes, so check your handbook, which will say what the car needs to regenerate the filter. Ignoring the light for too long can lead to a much bigger and more expensive problem.

What a DPF does

A diesel particulate filter sits in the exhaust and traps the fine soot that diesel engines produce, stopping it being breathed in by people around you. Most diesel cars built in roughly the last fifteen years have one.

Because it traps soot, the filter slowly fills up. To stop it blocking, the car burns the soot off at high temperature. This is called regeneration, and it is where most DPF problems start.

How regeneration works

Passive regeneration happens naturally when the exhaust gets hot enough during longer, faster driving. The soot simply burns away while you drive, and you will not notice a thing.

Some cars also use a fuel additive stored in a separate tank, which helps the soot burn at a lower temperature. On those cars the additive needs topping up at set intervals, and running it empty can stop regeneration working properly. It is also important to use the correct low-ash engine oil for a DPF-equipped diesel. The wrong oil can leave ash in the filter that regeneration cannot burn off, which shortens the life of the filter.

Active regeneration is when the car's computer decides the filter is getting full and raises the exhaust temperature itself, often by injecting a little extra fuel. You might notice the cooling fan running after you park, the idle speed being higher, a hot smell, or stop-start not working. If you switch off halfway through, the process is interrupted and the soot stays put.

Why short journeys cause problems

If most of your driving is short trips, such as school runs, popping to the shops or crawling through town traffic, the exhaust never gets hot enough for long enough. Regenerations get started and cut short, and soot keeps building up.

Around Walsall, a regular run on the M6 or a longer A-road trip every week or two can make a real difference for diesel owners who otherwise only drive locally. If your driving is almost all short hops, it may be worth thinking about whether a diesel suits you when you next change car.

What to do if the light stays on

If a steady drive does not clear the light, do not keep ignoring it. As the filter blocks, the engine may go into limp mode with reduced power, fuel economy gets worse, and eventually a forced regeneration or filter clean may not be enough, meaning the filter needs replacing.

If the car does go into limp mode, drive gently and get it to a garage rather than trying to force a regeneration by driving hard. Hard driving on a heavily blocked filter can make things worse, and some cars will not attempt a regeneration at all once the soot level passes a certain point. At that stage the filter needs a proper diagnostic check.

DPF problems are often a symptom of something else. Faulty sensors, a sticking EGR valve, injector problems, the wrong engine oil or a low additive level on cars that use a fuel additive can all stop the filter regenerating properly. Fixing the cause matters, or the filter will simply block up again.

Removing a DPF is not a fix. It is illegal to drive a car that should have a DPF without one, and a missing or tampered filter will fail the MOT. Visible smoke of any colour from the exhaust of a car fitted with a DPF is also an MOT fail.

Get your DPF checked

We can read the fault codes, check how full the filter is, test the sensors and related parts, and carry out a forced regeneration where it is appropriate. Most importantly, we look for the reason the filter blocked in the first place.

If your DPF light is on, contact MF Autocentre. Call 01922 636820 or book online.