A UK vehicle management company is claiming that some diesel engines fitted with particulate emissions filters can cause "serious operational issues" for fleets.

Lombard Vehicle Management said monitoring of the fleet it runs has shown that problems can occur if diesel particulate filters (DPFs, also referred to as FAPs by French manufacturers) are not cleared out when they become blocked, a process known as regeneration.

Regeneration requires the vehicle to be driven at a minimum speed for a period specified by the vehicle manufacturer - typically 50mph (80km/h) for around 20 minutes.

If filter systems are not cleared, vehicles can go into 'limp-home' mode or, in a worst-case scenario, break down altogether, Lombard said.

It said that instructions and warnings covering the use of the filters are usually detailed in the vehicles' handbooks but drivers and fleet managers may be unaware of the problem.

"The issue is particularly serious for fleets which spend a large amount of time in urban environments where stop-start conditions can result in time off the road due to lack of regeneration," the fleet manager said.

DPFs are not always required for diesel engines to meet the current Euro IV emissons standard but are considered the most likely method of achieving the reduced emissions required by the new Euro V standard due to come into force across Europe in 2008.

"We fully support the efforts made by vehicle manufacturers and fleet managers to be environmentally responsible, but a DPF is not always necessary to meet the Euro IV standard," said Lombard Vehicle Management head of sales Edward Pigg.

"With these new findings in mind, fleet managers and operators should consider and qualify vehicle usage very carefully before committing to a specific model, and should identify such vehicles already on their fleet and educate the drivers."