With modern diesel engines and the pursuit of ever cleaner engines, the car manufacturers have fitted a thing called a DPF (diesel partiticulate filter)to the exhaust system. This filter then reduces the emmisions from the exhaust.

However, if the exhaust system is not subject to a certain temperature for a sustained time them they soot up.

Driving a diesel car around town most of its life would not be enough to raise the temperature enough, hence the DPF would then block up, which would then reduce engine performance.

The only way to ensure that the DPF works correctly is to drive the car at motorway speeds for at least 30 mins - this would burn of the soot from the filter.

So if your nephew is a driver that doesn't use motorways much then he should not have purchased a diesel engine car but a petrol.

I would not have thought that he could blame the manufacturer, as he should have researched before he purchased.

So his option right now would be to use the car every so often on a good long run at motorway speeds or to change to a petrol engine car.