Motoring experts are urging car companies to come clean over the drawbacks of green technology fitted to diesel cars.
Filters are fitted to most modern diesel cars to prevent emissions of soot - but they can make the cars unsuitable for drivers who do most of their driving in towns.
Chris Nicols from Danescourt in Cardiff spent £14,600 on a second hand diesel Mazda 6 from Shires Mazda in Taunton – but had to get the oil changed after three months.
Chris, who had bought the car partly to avoid maintenance bills, told X-Ray, “I was gutted, frankly. I was very, very disappointed”.
Chris discovered that the oil level in the car was rising because of the way the diesel particulate filter cleans itself out. The cleaning process – known as “regeneration” – can only happen if the car is driving at constantly high revs for between ten and 15 minutes.
But as Chris and his wife use the car mainly for short journeys around Cardiff – the filter never regenerated. That led to diesel getting into the oil and the oil level rising.
It is feared that in some cases this could potentially lead to serious problems.
Mehdi Rafia from Essex told X-Ray of a serious incident in his diesel Mazda 6. His car began accelerating by itself - reaching speeds of over 110mph before he was able to turn off the engine and bring the car to a halt.
Mechanics told him the engine oil level had been excessively high. Mehdi believes the high oil level was caused by the diesel filter failing to regenerate.
Mazda say the oil level was too high but also that safety systems are in place to prevent this kind of incident. They've not yet established exactly what caused the incident, but say motorists do need to check their oil levels.
Back in Cardiff Chris says he was never warned that urban drivers might have problems with modern diesels. Shires Mazda in Taunton told X-Ray that because Chris had researched the car on the internet they thought he knew all about the problems. Mazda say all dealers are trained to question customers buying a diesel about their driving habits.
Tim Shallcross of the Institute of Advanced Motorists told X-Ray the way manufacturers and dealers were selling diesel cars was, “quite ridiculous”
He said, “They need to be more honest with people about the potential drawbacks of technology as well as the advantages.
“They need to ask a few more questions of people - so if someone's out shopping for a small car in a dealership, they should be asking what are you going to use the car for...how are you going to use it?
“It's not a question of losing a sale - there are perfectly good small petrol alternatives, which won't give any problems in town.”
Shires Mazda say there is nothing wrong with Chris’s car – but they have offered to buy the car back. However, they are offering £5,000 less that Chris paid for it in May.
Diesel Particulate filters are fitted to most new diesel cars since 2009 because of EU environmental legislation.