A blocked diesel particulate filter usually announces itself with a warning light and a car that will not clear it on short trips. Here is what is happening, what can be done about it, and why having the filter removed is not one of the options.
What a DPF does
A diesel particulate filter sits in the exhaust and traps soot before it leaves the car. It is not a consumable in the way an oil filter is — it is designed to clean itself out over and over, by burning the trapped soot off at high temperature. That process is called regeneration, and almost every DPF problem is really a regeneration problem.
Passive regeneration
On a long run at steady speed, the exhaust gets hot enough to burn the soot away on its own. The driver notices nothing at all. This is how the system is meant to spend most of its life.
Active regeneration
When the car has not had that kind of run, the engine management forces the issue: it injects extra fuel to raise exhaust temperature deliberately. You may notice the idle change, the cooling fans running, a hot smell, or the fuel consumption worsening for ten or fifteen minutes. If you switch the engine off partway through, the cycle is abandoned and the soot stays where it is.
Why filters block
- Short journeys. A car used for a few miles at a time never gets hot enough for passive regeneration, and keeps interrupting the active ones.
- A fault stopping regeneration. A failing differential pressure sensor, exhaust temperature sensor, EGR valve or turbo problem can prevent the cycle from starting or completing. The filter then blocks because of the fault, not because of how the car is driven.
- Leaking injectors or worn engine internals. Both put more soot and oil into the exhaust than the filter was designed to handle.
- The wrong engine oil. Low-SAPS oils exist because ordinary oil leaves ash the filter cannot burn off.
The symptoms
- a DPF or engine management light, sometimes both;
- reduced power, or limp mode;
- repeated active regenerations that never seem to finish;
- a noticeable drop in fuel economy;
- a hot, acrid smell after a drive;
- the engine oil level creeping up, as abandoned regenerations wash unburnt fuel into the sump.
That last one matters more than it sounds. Fuel diluting the oil shortens the life of everything the oil protects, so a DPF left blocked for months can turn into a far larger bill.
Start with a diagnosis, not a cleaner
A DPF light does not prove the filter is full. It can equally mean a sensor is reporting nonsense, or that something else is blocking regeneration. Reading soot and ash load, differential pressure, exhaust temperatures and the regeneration history tells you which of those you are dealing with — and whether cleaning the filter would fix anything or simply reset a countdown to the same fault. Our vehicle diagnostics service exists for exactly this kind of question.
The realistic options
| Option | When it suits | What it will not do |
|---|---|---|
| Forced regeneration | Soot load high but the filter still serviceable, and the underlying cause understood | Remove baked-on ash, or fix whatever stopped regeneration happening |
| On-car cleaning | Moderate blockage, filter sound | Rescue a filter that is cracked, melted or full of ash |
| Off-car cleaning | Heavy soot and ash where the filter is otherwise intact | Repair physical damage to the substrate |
| Replacement | Cracked, melted or collapsed substrate, or ash beyond cleaning | Stay clear if the fault that destroyed the first one is still there |
Carbon build-up elsewhere in the intake and on the injectors often goes hand in hand with a struggling filter, which is why engine carbon, DPF and fuel system cleaning are usually discussed together rather than one at a time.
Why removal is not an option
This is the part worth being plain about. Removing a DPF from a car that left the factory with one is illegal to then use on the road.
- Under the Road Vehicles (Construction and Use) Regulations it is an offence to use a vehicle that has been modified so it no longer meets the emissions standards it was designed to meet. Government guidance states that removing a DPF will almost inevitably breach those requirements.
- The penalties quoted by government are up to £1,000 for a car and up to £2,500 for a light goods vehicle.
- The MOT exhaust inspection has included a check for the filter since February 2014. A missing DPF, where one was fitted when the vehicle was built, is an MOT failure, and DPF tampering was added as an inspection item in the 2018 manual changes.
It is true that the current smoke test does not always detect a removed filter on a modern engine — DVSA has consulted on replacing it with more sensitive particle number testing, though that has not come into force. But "hard to detect" is not the same as lawful, and it does nothing about the declaration you would have to make to an insurer or to the next owner.
Keeping a healthy filter healthy
If the car is used for short trips, give it a longer run at sustained speed now and again, and let an active regeneration finish rather than switching off halfway. Use the oil specification the manufacturer asks for. And treat a DPF light as something to investigate rather than something to live with — see also what to do when your check engine light comes on.
Mitsiland works on diesel engines across Land Rover, Jaguar, Audi, Porsche, Bentley and Mitsubishi at our workshop in Denton, Manchester. If a DPF light has appeared, call 0161 320 7788 and we will start by finding out what is actually wrong.
