The Citizen's Advice Bureau (CAB) says it has had more than 70,000 complaints about used cars in the last year.
It's the first time it has collected data showing the full extent of the problem.
The CAB says it's by far the most complained about issue.
Newsbeat's been to meet 24-year-old James Elliott, a mechanic from Coventry.
He gives us a 60-second guide on what to look out for when buying a used car.
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