DPF regeneration

Diesel particulate filters have been mandatory in all diesel cars since 2009 and are used to reduce tailpipe emissions. The filters are designed to stop soot and carbon particulates from polluting the air. As with any kind of filter, DPFs have a finite capacity and the soot either needs to be burned off or the filter emptied and cleaned, this is called DPF regeneration.

DPF regeneration is a way to cleanly burn off the soot as it leaves a small amount of ash which is stored in the DPF until it is replaced. After a DPF has regenerated it will be more efficient and continue to minimise the amount of harmful pollutants that are released into the atmosphere.

Any issues with a DPF will lead to an MOT failure. Removing a DPF is illegal and will lead to an MOT failure, invalidated insurance and warranty and a fine of up to £1,000 for a car and up to £2,500 for a van.

What is a Diesel Particulate Filter (DPF)?

All exhaust fumes travel through a DPF before being released into the atmosphere. The DPF is found close to the engine so it can heat up enough to cleanly burn off the particulates it captures. The DPF stops these harmful particulates from escaping as they can cause breathing problems and pollution-related health issues. Some petrol cars also have similar filters which are known as gasoline particulate filters (GPF).

A telltale sign of an older diesel engine is black smoke coming out of the exhaust when the vehicle accelerates. DPFs prevent this from happening by capturing all the nasty pollutants that would otherwise be released.

They do however have a limited capacity and need to undergo DPF regeneration regularly to stay in good condition. Cared for properly, a DPF can last for over 100,000 miles.

DPF light

If the DPF is blocked or faulty, a yellow warning light will come up on the dashboard. There are two types of regeneration that can help to unblock the filter. Passive regeneration happens when the vehicle is travelling at high speeds, for example on the motorway for between 30 to 50 minutes. This means the engine will be hot enough to help the DPF heat up so the soot can be burned off, hence unclogging the filter.

Active regeneration can happen if the car travels at speeds over 40mph for around 10 minutes. On newer diesel vehicles active regeneration can be initiated by the vehicle approximately every 300 miles, by injecting more fuel into the engine so it gets hotter and heats up the DPF. This will lead to higher fuel consumption while the regeneration process is happening but it will return to normal once the process is complete.

If the journey is too short or the vehicle does not have enough fuel the DPF regeneration process might not work. To allow the DPF to regularly regenerate it’s advised the fuel tank is more than a quarter full, otherwise, the vehicle will opt to save fuel instead of redirecting it for DPF regeneration.

After trying to passively or actively regenerate the DPF, if the yellow warning light persists or turns red then the DPF requires immediate attention to avoid any damage. Don’t ignore DPF warning lights, if the DPF is blocked it can damage the engine and prevent it from starting. Ignoring warning lights could result in the DPF needing to be cleaned or replaced, which can cost anywhere from £1,000 to £3,500, plus labour.

How to avoid DPF problems

The first way to avoid DPF issues is by choosing the right car for your driving habits. If the vehicle will mainly be used for short journeys around town or spend lots of time sitting in traffic then a petrol, petrol-hybrid or electric car might be better suited to this kind of driving.

For regular long motorway trips, a diesel engine will offer good fuel economy and the DPF will passively regenerate on these high-speed journeys. If longer journeys at high speeds become less frequent, keep in mind ways to encourage active regeneration.

As well as taking care of the DPF itself, always make sure to check the vehicle handbook for the correct type of engine oil the vehicle requires. Some oils contain additives that can block the DPF so it’s always best to check what’s suitable for your vehicle.

As soon as the DPF light shows red the vehicle needs to be taken to a mechanic to assess the DPF and prevent any further damage. Regular servicing and frequent long journeys should ensure the DPF stays in good condition. A DPF that has been well looked after should last for over 100,000 miles.

When buying a diesel car with around 100,000 miles, make sure the vehicle has a full service history and ask a mechanic to check the condition of the DPF. Buying an older diesel car with a DPF that has not been sufficiently maintained could result in a hefty bill to replace the DPF, which could cost more than the value of the vehicle.