What The DPF Warning Light Means, and What To Do

What The DPF Light Is Telling You

A diesel particulate filter (DPF) sits in the exhaust and traps soot so it does not come out of the tailpipe. The filter has to be emptied from time to time by burning that soot off, a process called regeneration. The RAC says the engine starts an active regeneration when the filter is about 45% full, and the AA says this usually happens every 300 miles or so and takes 5 to 10 minutes.

The warning light, usually an amber box with dots inside it, comes on when those regenerations have not been able to finish. That is most often because the car has only done short, slow journeys. It does not mean the filter is ruined. It means the car is asking for the right kind of drive, and if that drive does not clear it, something is stopping the filter from cleaning itself.

The Messages Different Makes Show

Many cars put a text message on the dash instead of a plain light, and the wording changes from make to make. Here are three, with what the maker itself tells you to do.

• Other makers use their own wording; your handbook will name it.
• A message that says do not drive, or a red light, is a stop, not a request to keep driving.

How To Do A Regeneration Drive

If the light is amber, the car drives normally and nothing else is lit, a regeneration drive is the first thing to try. Pick a road where you can hold a steady speed without stopping: in our area that means the M62, the M57, the M58 or the East Lancashire Road rather than city streets. This is a prompted regeneration, so the maker's instruction applies. It is shorter than the 30 to 50 minute run at sustained speed that the RAC says passive regeneration needs; that longer run, every week or two, is what stops the light coming back.

• Put some fuel in first. Volkswagen asks drivers to refuel if the low fuel warning is on, because some cars will not regenerate on a nearly empty tank.
• Drive for the time your handbook gives, usually 15 to 25 minutes at a steady speed, and use a lower gear if needed to keep the revs around 2,000 or a little above.
• Do not switch off halfway. An interrupted regeneration has to start again, and Vauxhall warns that repeated interruptions can damage the engine.
• Expect some signs that it is working: the RAC lists cooling fans running on, a higher idle, stop/start switching off, higher fuel use and a hot smell from the exhaust.
• When you stop, do not park over dry grass or leaves. Ford warns the exhaust runs hot enough during regeneration to be a fire risk.

When To Stop Driving

A regeneration drive only helps if the system is able to regenerate. Volkswagen tells owners to get the car checked if the lamp is still on after about 30 minutes of the right driving. At that point more driving will not fix it, and pushing on can make the blockage worse.

The AA warns that ignoring the light and carrying on with slow, stop-start driving lets soot build up until the car drops into restricted performance mode, often called limp mode. We saw exactly that on a Mercedes Sprinter van in St Helens: two of its three exhaust temperature sensors had stopped working, so the filter could not regenerate, and the owner kept driving until the soot load reached 120% and the van went into limp mode.

• The light stays on after a proper 20 to 30 minute run
• The light flashes or turns red
• The engine management light comes on as well
• The car loses power, will not rev past a certain point or goes into limp mode
• You see smoke from the exhaust or smell burning

What Not To Do

Do not clear the light with a cheap code reader and carry on. The soot is still there, and so is whatever stopped it burning off: short trips, a faulty pressure or temperature sensor, an EGR problem adding soot, or the wrong oil (DPF engines need low-ash oil). Do not keep doing short school runs and hope it sorts itself out, because that is the driving that caused it.

Do not have the filter removed from a road car to make the light go away. The MOT has checked that a factory-fitted DPF is present since February 2014, and the MOT inspection manual lists evidence that the filter has been tampered with as a major defect. The RAC notes owners can also be fined up to £1,000 for a car and £2,500 for a van.

When To Call A Mobile DPF Specialist

Once a regeneration drive has failed, the next step is a diagnosis, not a guess. Voytech Tuning's mobile DPF cleaning comes to the car on your drive, at work or on the street across Liverpool, Manchester and the North West. Every visit starts with diagnostics: DPF fault codes, pressure and soot readings, and checks on the sensors and other faults that can stop a regeneration. If a clean is the right fix, it is a high-pressure chemical clean, followed by a recheck of the readings and advice on anything else we found.

Mobile DPF cleaning starts from £195, including the diagnostics and the recheck, and the average time on site is 45 to 60 minutes. Same-day bookings are available, subject to availability.