DIESEL fleets could be hit by increased servicing costs and fines from roadside emissions checks because current testing procedures are not accurate enough, the AA has warned. The breakdown service says equipment used by MoT stations and roadside test centres, which offers a snapshot measure of smoke density, is an inaccurate compromise because intricate analysis would be too expensive and time consuming.

John Stubbs, head of technical policy for the AA, said: 'The current testing can pick up a badly tuned engine, but it does not give the whole answer as to how an engine is performing. An engine could fail a test, but be perfectly fine out on the road. There may just be slight carbon deposits on the fuel injectors or the vehicle may not be warmed up thoroughly that causes a test fail.'

But he said a more accurate test would be a long way off: 'There is a lot of head-scratching going on at the moment. A truly accurate test would cost too much and be too expensive and the vehicle would have to be run over a very long time, which is not feasible.'

One fleet has reported that in-house checks using the same equipment as MoT centres has revealed that 50 of its fleet of 600 1.8 N and P-reg turbodiesel Rover 200 and 400 models have illegal emissions. But it is certain the problem lies with the cars and not the equipment. A Rover spokesman insisted the company had not heard any comments from fleet or private motorists of emissions problems with its vehicles. He added: 'We will work with the company to work out why they are having this problem.'