A spot-check of garages in Leics has found the vast majority failed to service cars properly and the prices charged varied greatly.
Officers from Leicestershire's Trading Standards Department took cars to garages all over the county for a standard service.
In eight out of 10 cases, vehicles were returned with faults like blown bulbs, torn wiper blades and deflated tyres.
Prices varied widely, with a spark plug costing between £2.64 and £10.33.
'Disappointing result'
Officers say they will now be investigating potential breeches of the Trade Descriptions Act.
They advised motorists to shop around to make sure they get the best deal.
David Bull, head of Leicestershire County Council's Trading Standards Service, said: "Having your car serviced is a necessity to ensure it is running safely and efficiently.
"It is disappointing that in a number of instances the cars were returned with faults remaining, problems not reported or items not checked."