DPF: Diesel Particulate Filter

A blocked DPF

Some filters are single-use, intended for disposal and replacement once full of accumulated ash. Others are designed to burn off the accumulated particulate, either passively through the use of a catalyst or by active means such as a fuel burner which heats the filter to soot combustion temperatures. This is accomplished by engine programming to run (when the filter is full) in a manner that elevates exhaust temperature or produces high amounts of NOx to oxidise the accumulated ash, or through other methods. This is known as "filter regeneration".

Cleaning is also required as part of periodic maintenance, and it must be done carefully to avoid damaging the filter. Failure of fuel injectors or turbochargers resulting in contamination of the filter with raw diesel or engine oil can also necessitate cleaning.

The regeneration process occurs at road speeds higher than can generally be attained on city streets; vehicles driven exclusively at low speeds in urban traffic can require periodic trips at higher speeds to clean out the DPF. If the driver ignores the warning light and waits too long to operate the vehicle above 40 miles per hour (64 km/h), the DPF may not regenerate properly, and continued operation past that point may spoil the DPF completely so it must be replaced.

Some newer diesel engines, namely those installed in combination vehicles, can also perform what is called a Parked Regeneration, where the engine increases RPM to around 1400 while parked, to increase the temperature of the exhaust.

DPF and the law

The UK Government announced a change to MOT testing from February 2014: MOTs for diesel vehicles were tightened to ensure that DPFs fitted as standard are still present. Garages and testing stations are required to check for a DPF in the inspection of the exhaust system as part of the MOT, and vehicles automatically fail if one is not present where it should have been fitted as standard.

This means the practice of removing DPFs for performance gains, or to bypass the cost of cleaning or repairing clogged filters, is policed more thoroughly. It is an offence to drive a vehicle if a DPF has been removed.

Having problems with your DPF?

Sometimes it is not possible to clean or repair a DPF because it has become too blocked. If this is the case we can obtain and fit a new DPF.

Frequently we find that when a DPF has been removed rather than cleaned or replaced, the back pressure that the turbo relies on is altered. This means that performance is seriously affected, which will lead to inevitable early failure.