How to Avoid DPF Problems
Ellie Dyer-Brown, 8 days ago
5 min read
- Exhaust
- How to
FixMyCar explains everything you need to know about the diesel particulate filter.
The diesel particulate filter (DPF) is fitted to the exhaust system to control emissions. Every modern diesel car must have one by law, yet many drivers aren’t aware that it can cause problems if it becomes blocked.
In this guide, we explain everything you need to know about diesel particulate filters, including what they are, how they work and how to get the best out of them.
Contents
How often does DPF regeneration occur?
How to tell if your DPF is blocked
How to clear the DPF warning light
How long can you drive with the DPF light on?
What is a DPF?
Diesel particulate filters capture and store soot from the exhaust to reduce emissions. Under the correct conditions, they are self-cleaning, so you don’t have to replace them frequently, unlike oil and air filters. This cleaning process is called regeneration.
Legislation was introduced in 2009 to make DPFs mandatory; all diesel cars produced since then have at least one filter, sometimes more.
Is it illegal to remove a DPF?
Removing a diesel particulate filter is illegal. If caught, you could receive a fine of up to £1,000 for a car and £2,500 for a van. It could also invalidate your insurance.
Do you need a DPF to pass an MOT?
The diesel particulate filter has been checked during MOTs since 2014. If you remove your car’s filter, it will automatically fail.
Another thing to consider is that sometimes, removing the DPF triggers a dashboard warning light, which is also an automatic fail.
This guide covers common MOT fails and how to avoid them.
What is DPF regeneration?
Regeneration happens when the filter reaches a hot enough temperature to cleanly burn off soot, reducing emissions and preventing the thick black exhaust smoke that diesel cars used to produce.
How often does DPF regeneration occur?
There are two types of regeneration - passive and active - which occur at different times.
Passive regeneration
Passive regeneration typically happens when you drive your car at motorway speeds, creating enough heat to clean the filter. However, the rev range your engine operates in is more important than the speed; passive regeneration works best when you keep your car in a lower gear to sustain a higher RPM.
One of the most significant drawbacks to the DPF system is that it is hampered by frequent short journeys, when the filter doesn’t reach a high enough temperature to clean itself. If this happens often enough, it can lead to a blockage. That’s where active regeneration comes in.
Active regeneration
Car manufacturers introduced active regeneration because many car owners don’t do the kind of driving required for passive regeneration. When the soot level reaches a predetermined level - usually around 40-50% - the engine control unit (ECU) injects more fuel into the system, which raises the exhaust temperature to burn off the soot.
During active regeneration, you may notice symptoms such as:
An increase in fuel consumption.
An increased idle speed.
A hot, unpleasant smell from the exhaust.
Cooling fans running.
Deactivation of the stop/start system.
How to tell if your DPF is blocked
A clogged diesel particulate filter should trigger a yellow dashboard warning light. It usually looks like a rectangular box with dots in the middle and pipes coming out of each end - you can check this information in your vehicle handbook. The engine management light may also appear.
In some cars, including Vauxhalls, Skodas and Peugeots, it's common for the glow plug warning light to switch on due to a DPF blockage.
Find out what common car warning lights mean in this guide.
In case a dashboard warning light isn’t triggered, here are the symptoms likely to accompany DPF failure:
Poor fuel economy
Loss of power (limp mode)
Unusual smells
A chugging engine
How to avoid DPF problems
DPF maintenance involves making sure the filter can regenerate. Taking your car on a lengthy motorway journey (at least 30 minutes) a few times a month while keeping it in a lower gear than normal is a good way to do this.
When your ECU triggers active regeneration, it’s important to continue driving long enough to complete a full cycle. Otherwise, the DPF warning light will stay on, indicating the filter is still partially blocked.
Other steps you can take to prevent problems include:
Adding a DPF cleaner to the fuel tank every so often.
Using high-quality fuel.
Following a regular servicing schedule, which ensures the fuel filter is replaced at appropriate intervals.
This guide explains what's included in a full and interim service.
How to clear the DPF warning light
When the DPF warning light is triggered, try to initiate a regeneration cycle by driving at least 40 mph for ten minutes in a low gear. If this doesn’t work and the light stays on or turns red, contact a mechanic as soon as possible to avoid causing further engine damage.
Some garages offer forced regeneration, which is sometimes an effective way of clearing the DPF. Although it isn’t a guaranteed fix, it’s cheaper than replacing the component.
According to FixMyCar’s data, a DPF clean costs £226.99 on average. Replacing the filter costs considerably more, with an average price of £472.30.
FixMyCar’s Matt Wrankmore explains why replacing a DPF can cost so much:
“A few different things affect the price of a DPF change. The first is that it’s part of the exhaust system, which gets extremely hot and dirty, so parts often get stuck. In some cars, the filter can be hard to reach, and the part is expensive for what it is - all these things drive the cost up.”
How long can you drive with the DPF light on?
When the DPF is blocked, it will often trigger ‘limp mode’, a safety feature that reduces a vehicle’s speed. This is a car’s way of protecting itself from damage.
You can continue for a short while (to get home if you’re nearby or reach a safe place to stop), but to protect your engine, you should avoid driving if possible and seek mechanical assistance.
Some models are programmed not to restart after a certain number of miles until the problem is fixed - you can consult your vehicle handbook for details.
Do DPF cleaners work?
DPF cleaners are effective to an extent, but they can’t work miracles. Even forced regeneration, carried out by professionals at a garage, doesn’t always work if the blockage is too severe.
Adding a DPF cleaner to your fuel tank every so often is a good preventative measure. Likewise, adding some to the system when the warning light is triggered may aid the regeneration process, but it isn’t guaranteed.
Once your car is in limp mode, the blockage is likely too severe for a fuel additive to be effective.
How does the EGR valve affect the DPF?
The exhaust gas recirculation (EGR) valve and DPF are closely linked parts of the exhaust. The EGR valve recirculates exhaust gases back into the system, where they pass through the DPF.
However, due to a build-up of carbon deposits, the valve can sometimes get stuck open, accelerating the accumulation of soot in the DPF until it becomes blocked.
Can a blocked DPF cause turbo failure?
Yes, a blocked DPF can cause problems with the turbocharger. The blockage prevents gases from moving through the exhaust system at the correct rate, creating back pressure and an increased temperature in the turbo housing. In turn, this pressure can cause oil and gas leaks and carbonisation of the oil within the turbo.
Turbo problems are notoriously expensive to repair, costing £1,125.99 on average, according to FixMyCar’s data. That’s why you should take DPF issues seriously when you notice the first signs of trouble - it could save you money in the long run.
Having problems with your car's DPF? FixMyCar's garage network can get you back on the road in no time. Enter your details to receive quotes from local mechanics.
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