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Diesel particulate filters – everything you need to know

At Bryan Swales Auto Repairs we’re keen to ensure you have trouble-free motoring in a car you can trust. We receive a fair number of enquiries for DPF faults so thought it’d be a good idea to give you the information you need to keep yours healthy. 

Diesel particulate filters (DPFs) have been known to cause some pretty hefty repair bills. If your DPF has never been clogged, you may not even realise it exists. However, when it does develop a problem, you’ll know about it!

So, what is a DPF, and why does it deserve your attention, and what do you need to do if your light is on?

What is a DPF?

When people refer to DPFs or soot traps, they are talking about a type of filter that both captures and stores exhaust soot to reduce the emissions from a diesel vehicle. 

When this happens it reduces harmful exhaust emissions and helps prevent that tell-tale black smoke that used to be so commonly seen billowing out of the back of diesel cars.

Whatever the car, the process is similar: exhaust gases move through the filter, large soot particles created by the combustion process are left behind in the DPF. These particles are what can cause the problem. The filter can become blocked with soot, which can stop the engine running and leave you with a repair bill that can easily run into the thousands.

DPFs only have a limited capacity, so to clear them out the soot has to be burned off at a high temperature, a by-product of this process is that ash is created and resides in the filter. The ash cannot be expelled from the system and accumulates over time. 

Common Causes of Failure?

Most issues with DPFs occur when this cleaning process, also known as ‘regeneration’, is interrupted.

To ensure the regeneration takes place, the majority of manufacturers suggest that the car is driven for more than 15 minutes at a consistent speed over 40mph every few hundred miles. The idea is that following this procedure should clear the filter.

When a DPF is not able to be regenerated passively, the car’s onboard computer has to take the necessary measures to prevent the DPF from clogging. When the computer detects that the filter is on its way to being blocked, it raises the temperature of the exhaust’s gases to begin the regeneration process. This then takes about between 10 and 15 minutes to finish. In this time it’s probable that you’ll notice it will deactivate with cars that have an engine stop-start system.

If this process is frequently interrupted because, say, you do lots of short, urban journeys and reach your destination before the regeneration has completed, the DPF warning light is bound to come on. When this happens, it is recommended that you immediately take the car out either on a dual carriageway or motorway for at least 15 minutes of continuous driving. This should give the DPF enough opportunity to regenerate. You’ll find the specific instructions for your vehicle in the driver’s handbook.

If the warning light is ignored and you continue driving at a slow pace in stop-start traffic, don’t be surprised when and if the other warning lights come on and your car goes into a limp-home mode as a way of automatically preventing additional engine damage.

When a car in this condition is left for too long, the DPF is not able to regenerate itself and will either need to be cleaned or replaced, additional diagnosis may also be required.

If the DPF does need to be replaced, the majority of manufacturer warranties will not cover the cost if the fault is found to have been caused by the driver’s driving style and not by the filter itself. In cases like this, you’ll potentially be looking at a massive four-figure bill.

Why is a DPF required?

The Euro 5 legislation that was introduced ten years ago to govern an exhaust’s emissions, and in particular CO2 emissions, had the effect of making DPFs mandatory. Since 2009, about one in every two new cars each year has been powered by diesel.

It’s now illegal to drive any car designed to have a DPF that doesn’t have one fitted because without a DPF your car will simply not meet its emissions standards.

Checking that a DPF is present became part of the MOT test procedure in February 2014. All cars designed to have a DPF now get inspected. Absence of a PDF means an immediate failure.

How To Avoid A Big Bill?

So… if your DPF light is on and remains lit after you’ve followed the instructions in your handbook for regeneration (driving on a dual carriageway etc), what next?

We’re keen to keep any remedial work to a minimum which is why an accurate diagnosis is essential.  There are a number of subsystems that are required for efficient regeneration (glow plugs, additive injectors etc), it’s very common for one of these to have caused the fault. Once we’ve found the root cause we can then assess if cleaning the DPF is suitable. 

At Bryan Swales Auto Repairs we have a lot of experience in DPF repairs. You’ll be guaranteed a high level of workmanship and great value.

Call today with you DPF faults. We’re here to help.

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