Keeping Your DPF Happy

Diesel Particulate Filter

If you own or drive a diesel vehicle, you’ll have heard of its diesel particulate filter, or DPF. Chances are, as a knowledgeable motorist, you have a good idea about its function and importance. If you’re a newer driver, however, or you haven’t had a diesel vehicle before, you might need a few pointers about your DPF so that you understand what it does and what you need to do to maintain it.

What is a diesel particulate filter?

Your DPF is a filter that traps and stores exhaust soot – some people call them soot traps – so these particles aren’t emitted into the atmosphere. Since 2009, the Euro 5 emissions laws made DPFs mandatory in an attempt to reduce CO2 emissions from vehicles.

These traps can get full and so your DPF needs to be “regenerated” every now and then to clean the filter. The regeneration process involves heating the filters to at least 600C to cleanly burn off all the soot.

Do I have to keep my DPF?

Yes. It’s illegal to remove your DPF and you can be fined up to £1,000 for cars and £2,500 for vans if you’re caught out. Removing your DPF can also invalidate your vehicle insurance.

How can I tell if my DPF is blocked?

If your DPF is blocked or otherwise faulty, you’ll see an orange light on your dashboard. Usually it’s an engine graphic with a pipe and dots, but styles vary between manufacturers, so always check your handbook. 

Why does a DPF become blocked?

The main culprit of a blocked DPF is making a lot of short, low-speed journeys. This doesn’t heat up the DPF enough to start burning off the accumulated soot.

Other causes include:

• A poor servicing history as a poorly-maintained DPF will fail sooner than a well-serviced one, which should last for at least 100,000 miles
• Using the wrong type of oil, as some oils contain additives which could block your DPF
• Making certain performance modifications or using low-quality fuel, and
• Running on an almost-empty tank, as your engine may “decide” it can’t spare the fuel to heat up your DPF (active regeneration, more about that later)

How do I maintain a diesel particulate filter?

The best thing you can do for your DPF is to make sure it can regenerate itself effectively when necessary (which is when the warning light appears).

You have two types of regeneration, passive and active.

Passive regeneration 

Passive regeneration happens when the car runs at speed for long enough for the exhaust system to heat up to at least 600C, which burns away the accumulated soot in the DPF. Motorway journeys are ideal for this, as they often involve 30-60 minutes at a sustained speed (at least 40mph). 

Active regeneration 

Not everyone gets onto the motorway often enough, so there’s also active regeneration. This is when extra fuel is injected into the filter once a preset limit of soot is reached. This extra fuel helps to burn away the soot. 

If your journey isn’t long enough, however – ten minutes or shorter – then the DPF might not fully regenerate and you may still see your warning light. 

Signs active regeneration is happening include:

• A change in the engine “note”
• You hear cooling fans
• A slight but noticeable increase in fuel consumption
• An increase in your idling speed
• Your automatic stop/start function is deactivated, and
• An acrid smell from the exhaust

When regeneration doesn’t work

If you can’t clear your DPF light, or if it turns from orange to red, you need to book into a local garage for a checkup. Replacing a badly-damaged DPF can get very expensive, so don’t delay.

Some garages can do a forced regeneration. This process should cost around £100, but it usually clears the DPF of soot, letting it do its job properly. Most DPF issues are caused by incomplete regeneration and if left, these issues could increase your exhaust emissions and put your car into a speed-limited limp mode. Even worse, some cars will simply refuse to start after a certain number of miles.

Can my DPF affect my MOT?

A DPF check became part of the MOT test in 2014, so if yours isn’t working properly or has been removed, you’ll fail.

How much do new DPFs cost?

A new DPF can cost anywhere between £1,000 and £3,500, so a premature replacement could wipe out your savings from using diesel. In older cars especially, the cost of replacing the DPF can outweigh the value of the car, so make sure you look after yours.

Help, advice and maintenance is available for all Diesel vehicles at all good garages.