Diesel particulate filter (DPF) cleaning or replacement

Soot burns, ash does not

Regeneration removes soot but not ash. Ash comes from the additives in engine oil, as a small amount of oil burns in the cylinders. In cars with an additive system, the fuel additive leaves ash as well. Ash cannot be burned off, so it builds up in the filter over the whole life of the car. Once a car has covered a lot of kilometres, the filter can clog with ash even if regeneration works perfectly. Then the only remedy is removing and cleaning the filter, or a new filter.

That is why a diesel with a particulate filter runs on a low-ash engine oil. The right grade is given in the service schedule, and it is usually an ACEA C class oil. The wrong oil fills the filter with ash faster.

How you notice a clogged filter

The most common sign is the particulate filter warning light on the dashboard. If the filter cannot clear itself, the engine warning light often comes on too, and the engine management may limit power to protect the engine. Other signs:

• the idle speed is higher than usual, or the cooling fan keeps running after the engine is switched off, which means a regeneration was running
• fuel consumption rises for no obvious reason
• an acrid smell around the car after stopping
• the oil level rises between services

A single interrupted regeneration is not a fault. Repeated interruptions show that the filter no longer has time to clear itself.

When cleaning is due

On many cars the particulate filter has no fixed cleaning or replacement interval: the control unit keeps track of how full the filter is. The filter fills according to how the car is driven, the engine's condition and the oil, and the difference between cars is large. A car driven mostly on short trips in town rarely gets to finish a regeneration. In winter the engine warms up slowly, and on a short trip a regeneration may not even start. A car driven on the open road every day, on the other hand, can go a long time without a cleaning.

If the manufacturer has set an interval for the filter, it is in the service schedule. In cars with an additive system, refilling the additive tank is part of the service schedule as well, and its interval varies by model.

Cleaning is due when the measured data show the filter is full and a service regeneration started with a diagnostic tool no longer brings the pressure difference down. That can only be seen by measuring, not from the warning light.

What happens at the workshop

1. We read the fault codes and the filter's measured data: the estimated soot load, the pressure difference and the regeneration history.
2. We check the sensors and hoses. A faulty differential pressure sensor can look like a clogged filter to the control unit even when the filter is fine, and a cracked or blocked sensor hose gives a false reading.
3. We look for the cause. A sticking EGR valve, a leaking injector, a turbocharger oil leak or a thermostat stuck open can make more soot than the filter can burn, or stop regeneration altogether.
4. If the filter holds mostly soot and not too much of it, we run a service regeneration with a diagnostic tool. The engine management then burns the filter clean under control, and the filter does not need to come off.
5. If there is too much ash, too much soot for a safe regeneration, or a service regeneration does not bring the pressure difference down, the filter is removed, cleaned and refitted with new gaskets.
6. We reset the control unit's learned values where the manufacturer requires it, and road test the car. On the road test we make sure the pressure difference stays normal and no fault codes return.

If a fault is behind the clogging, it has to be repaired. Otherwise the cleaned filter is soon full again.

How long it takes

Removing and refitting the filter takes anything from a few hours to a full working day, depending on the car. On some cars the filter can be reached from below, on others it sits behind the engine, and pipes, brackets and heat shields have to come off first. The cleaning takes time on top of that, so the car may stay at the workshop until the next day. If a service regeneration clears the filter, it does not have to come off and the car is usually ready the same day. We tell you the schedule once we have seen the car.

What affects the price

• Where the filter sits. That decides how much of the car has to come apart.
• Seized bolts. Exhaust bolts are often seized after years of heat and road salt, and removing a broken bolt adds work.
• The cause of the clogging. A faulty EGR valve, injector or sensor is a repair of its own.
• The filter's condition. A cracked or melted core cannot be cleaned. Then a new filter is needed, and its price depends on whether you choose an original part or an aftermarket one.
• The additive system. If the car has an additive tank, the additive level is checked and the tank refilled if needed at the same time.

We check the car and tell you the price before the work starts. If the check turns up anything else, we tell you that first as well, and you decide whether it is done.

The particulate filter and the inspection

A particulate filter fault can show up at the inspection in two ways. A cracked filter lets soot through and raises the exhaust measurement result, while a clogged one shows up as a fault code and a lit warning light. In Traficom's inspection statistics for 2025, the diesel exhaust measurement accounted for 5 778 rejecting faults and the on-board diagnostics (OBD) for 30 779, all fuel types included.

Simply clearing the fault code before the inspection fixes nothing. The code comes back the next time the filter fails to clear. If the inspection is close and the particulate filter light is on, bring the car to us first.

What you can check yourself

• The oil level. Read the dipstick on level ground after the engine has been off for a while. If the car has no dipstick, the level is usually shown in a menu on the instrument cluster or the centre screen. If the level rises between services, fuel from interrupted regenerations has most likely mixed into the oil.
• The owner's manual. When the particulate filter light comes on, see what the manufacturer advises. Many manuals advise driving at a steady speed on the open road for about a quarter of an hour so the filter can clear. The speed and the time vary by make.
• Interruptions. If you notice a regeneration is running, do not switch the engine off straight away: drive on for a while so the regeneration can finish.
• The oil grade. Make sure the service uses the oil the service schedule specifies.

When not to drive

Stop and call if the engine warning light flashes, the car drops into limp mode mid-journey and loses power, the exhaust pours smoke, or a burning smell comes into the car while you drive. A clogged filter raises the pressure on the exhaust side, and regenerating a very full filter can heat the core until it cracks or melts. At that point cleaning is no longer enough.

If the oil level is clearly above the maximum mark, do not drive until the oil has been changed and the cause found. Oil diluted with fuel does not lubricate properly.