Diesel cars are less reliable and more expensive to fix than their petrol counterparts, according to a study by MotorEasy.

It analysed repairs of 30,000 three to eight year old cars over a 12-month period and found diesel models broke down three times more, on average.

Drivers of diesel models were also hit with bigger repair bills. The average engine repair bill for a diesel was £517, compared with £433 for a petrol model, a difference of 20%.

Duncan McClure Fisher, founder of MotorEasy, said: “Diesels experience many more small problems than petrol cars. They are less reliable and, when a big item goes wrong, it costs a lot more to put right."

He advised: “If you’re still considering a used diesel car, our advice is to avoid high-mileage examples, particularly if you are only driving low mileage or doing city driving.”

Engine faults were found to be far more prevalent in diesels than petrol cars, the biggest repair cost found by MotorEasy amounted to £4,030.80.

Of the brands studied, Alfa Romeo, Land Rover, Kia and Mazda diesels have the highest average diesel engine repair cost versus petrol counterparts.

Least reliable diesel cars by make

Make

Average diesel failure rate

Alfa Romeo

47%

Land Rover

41%

Mitsubishi

36%

Saab

33%

Mazda

33%

Kia

29%

Mercedes

27%

Vauxhall

26%

Mini

26%

Audi

26%

 

The higher rate of diesel engine failures is mostly down to the fact that diesel engines are under more pressure than petrol counterparts. Diesel units rely on self-compression, meaning that fuel is compressed to a much greater extent, putting more pressure on internal engine parts.

Only seven of 25 car makers analysed showed fewer breakdowns for diesel models – petrol-powered BMWs experienced faults 60% more often, while Audi, Skoda, MINI and Peugeot also bucked the trend.