Recent services on certain cars by Warwickshire garages were performed to a poor standard, Warwickshire County Council has claimed.
Warwickshire Trading Standards invited motorists who had their car serviced to get them checked again for free in Warwick at the end of January.
But just one out of the five cars checked had been serviced correctly, with customers paying an average of £306 for the job.
County councillor John Horner, portfolio holder for community safety, claimed many drivers depended on garages to service their cars correctly.
He said: “Most consumers are not car experts - they rely on their garage to carry a proper service so their vehicle remains both safe and roadworthy.”
Trading Standards’ vehicle examiner carried out lengthy and thorough checks on all the cars.
He found some work had been carried out incorrectly, whilst other issues that should have been reported to the owner had not.
Problems included failing to notice faults on the steering mechanism and two cases where engine oil had been over-filled, which can cause engine damage, whilst failing to top up the coolant, which can lead to overheating.
The council confirmed Trading Standards will work with garages to keep quality up and will take action against garages that consistently fall below what is expected.