What is a DPF?

Diesel Particulate Filters (DPF) – or soot traps – are more common on the roads today due to ongoing changes in emissions regulations. With Euro 5 standards now with us, Particulate Filters in Diesel car exhausts are now as common as catalytic converters on petrol cars.

This relatively new way to combat vehicle pollution has proven to be very successful in providing major reductions to diesel exhaust emissions, however, as with any new technology, sometimes there can be problems. Evidence of these systems failing to regenerate comes to light every day. In the UK, the AA is getting more & more calls from drivers regarding the DPF light illuminated on the vehicle dashboard indicating a filter blockage, not knowing what it is or what to do about it.

How Do Diesel Particulate Filters Work?

As with any filter, they ‘filter’ or trap particles, in this case harmful diesel exhaust soot particles, so they have to be emptied regularly to maintain performance. The DPF needs to be cleaned regularly, through a process called regeneration, either active, passive or forced regeneration, the accumulated soot is burnt off at high temperature (around 600°c) to leave only a residue of ash, effectively renewing or regenerating the filter, ready to take on more pollution from the engine.

Why does my Diesel Particulate Filter Block?

Every vehicle type & engine combination can have differing reasons as to why the filter blocks. The rate of particulates generated by the engine oil, the quality of the fuel, driving style, or even the location of the DPF in the exhaust system can all contribute to the filter blocking or not regenerating fully.

Generally, the problems arise in and around town stop start driving where the regeneration process might not complete. A warning light will illuminate or a message indicating the DPF is full displays on the dash. If you continue to drive in the same manner, the soot build up will increase until other warning lights illuminate and the vehicle will go into ‘limp’ mode, where driving speed is restricted.

This warning stage will now involve a visit to the mechanic to carry out a forced regeneration on the filter, where filters that cannot be regenerated, will then cost you between £1500 – £4500 to replace the filter.

• 100% Diesel Bio Fuel: Using these Bio Fuels can also contribute to extra soot build up loading in your DPF as the Bio Fuel may not burn as ‘clean’ (produce more particulates) as your regular Diesel fuel.
• City Cycle driving: We don’t all use our cars in the same way, if you only use your car around town you may experience a faster build-up of soot in your DPF as the regeneration process may not complete in short city or around town driving.
• Temperature: The DPF relies on temperature to carry out a filter regeneration, at around 600c, so a lot of short trips, low speed driving will not provide the exhaust system with a high enough temperature to begin or complete a regeneration, so the filter can block up faster.
• High Kilometre vehicles: As these vehicles with DPF systems age they will start to show that filter regeneration is harder to complete. Like any part on the car they do wear out and can no longer be repaired.

What should I do if my DPF light comes on?

Follow vehicle manufacturer’s handbook on procedure to unblock the DPF. Immediately contact AutoLogic for help and advice.

If you ignore the light and keep driving in a relatively slow, stop / start pattern soot loading will continue to build up until around 75% filter capacity, then you can expect to see other dashboard warning lights illuminate too. We see more evidence of these systems failing to regenerate, even on cars used mainly on motorways. It seems that cars with a very high sixth gear engine rev are too low to generate sufficient exhaust temperature.

MOT Requirement

As of 16th February 2014 Section 7.1 of the inspection manual will be amended to include a check for the presence of catalysts and particulate filters on diesel powered vehicles. Any vehicle where a catalytic converter or particulate filter is missing where one was fitted as standard will fail the test.