A taxi owner was forced to spend almost £4000 after problems arose with a diesel emission filter that is becoming a real issue for the taxi industry.

Two hearings over suspension of taxi vehicle licences were heard at South Ayrshire Council’s regulatory panel on Thursday.

South Ayrshire taxi owners Graham Campbell and David Kerr had seen vehicles fail emissions tests which were eventually narrowed down to issues with equipment designed to capture harmful particles.

The Diesel Particulate Filter (DPF) – sometimes known as a soot trap - captures the larger particles from diesel emissions.

However, the filters have a finite capacity and must either be ‘burned off’ through a process called regeneration or eventually replaced.

Regeneration generally happens on longer, higher speed journeys on trunk roads and motorways.

However, the shorter, lower speed journeys undertaken by taxis, mean that the temperature needed to regenerate is not met and increases the likelihood that they will get clogged with soot.

Graham Campbell had one of his Citadel Taxi vehicles fail emission tests.

Mr Campbell was represented by Tom Getgood, of South Ayrshire Taxi Owners Association, who said the taxi had successfully passed a preliminary test for emissions and said that the problem arose during council’s taxi inspection.

Mr Getgood said Mr Campbell had cleaned and carried out a ‘regeneration’ of the diesel filter. However, despite passing the pre test, the vehicle produced excess smoke on being revved during the inspection.

A new DPF, costing £900 plus fitting, sorted the problem.

Mr Getgood said: “This was an intermittent fault and was unforeseeable. He takes great pride in his fleet of taxis and has nine people in employment.”

Council fleet inspector Matt Wright accept the explanation, saying that an "intermitent fault is hard to find as it can appear and disappear".

The second hearing also came down to the impact of DPFs in taxis.

One of Mr Kerr’s vehicles had been refused over exhaust emissions. Having been advised to replace the injectors,  he found that the engine had began to make ‘knocking’ noises.

A mechanic recommended a new engine, but after this was installed and the car was deemed to be running well, it failed on emissions a second time.

Only once the DPF was finally replaced did the vehicle get the all-clear.

Mr Getgood also represented Mr Kerr. He told the panel that the car had been presented for a pre-test at Arnold Clark and had passed.

Mr Getgood explained: “The next day, there was a change in the revs. The engine was basically running away.”

At this point the mechanics recommended replacing the injectors, but Mr Kerr was then faced with ‘knocking’ in the engine.

The mechanic then recommended replacing the engine. Mr Kerr did so and, with the new engine, the vehicle seemed to be running well, only to fail on emissions a second time.

It was finally resolved when he bought a new DPF.

Mr Getgood added: “So that was £900 for the DPF, £1,450 for the engine and £1,600 for fitting.

“It has been one of those things, trying to find the fault that went on and on. He’s never had mechanical problems with vehicles before.

“There was nothing left this man could do to try and keep up. He couldn’t do any more to fix it."

Councillors were asked whether to suspend the vehicle licences, but in both instances agreed to take no further action.

Councillor Ian Cavana (Labour, Ayr North) said: “The gentleman has spent enough money. The figures there are quite frightening actually."

Councillor Mark Dixon (SNP, Ayr North) asked Mr Wright whether the issues around diesel engines, such as the DPF, were becoming apparent in the taxi industry.

Mr Wright replied: “It is becoming apparent worldwide, certainly more so in the UK because of the stop-start nature of driving here.

“Taxis do more stop-start and mileage than any. It is the mileage that blocks the system.

“You ultimately pay for that by paying £1,000 for a DPF.

“They are really not suited to that [short journeys]. But we have to live with that. Not ideal for taxi owners.”