Buyers are being warned to check their rights ahead of purchasing a used vehicle.
The warning from the Office of Fair Trading (OFT) comes after an increase in complaints about second-hand cars.
The OFT said during the first-half of the year, complaints about used cars rose by 18% in comparison with the same period last year.
The advice service, Consumer Direct, registered more than 38,000 complaints about second-hand cars bought from dealers.
Every year, about 3.6 million second-hand cars are purchased, with buyers spending a total of £24 billion.
Research showed many car owners are forced to fix faults that the dealer should have corrected, costing an average of £425.
Consumer Direct operations manager Michele Shambrook said: "Cars are an expensive purchase, so before parting with any money people need to know exactly what they are getting and what they can do if things go wrong."
The OFT is urging buyers to consider a number of factors - including the car's mechanical history and mileage, the number of former owners, documentation about the full service history and modifications - before going ahead with a purchase.
© Press Association 2010