An undercover investigation into UK car garages suggests the vast majority miss simple mechanical faults, consumer watchdog Which? has said.
Which? took cars with four deliberate faults to 62 firms including both franchises and independent sites.
Some 87% missed at least one problem, such as a deflated spare tyre or low brake fluid. All the faults were serious enough to cause a crash.
A number of codes of practice have been introduced to improve standards.
The Retail Motor Industry (RMI), an organisation which represents companies across the country, said although it was "disturbed" by the results, the survey represented a small sample when compared to the number of businesses that make up the whole industry.
However, Which? called the latest investigation a "worrying snapshot of an industry that desperately needs to clean up its act".
Only eight garages fixed all of the problems on the vehicles, which were between three and six years old, while five did not rectify a single fault.
Which? found 48% of garages failed spot brake fluid was at the minimum level; 68% did not inflate the nearly flat spare tyre; 57% missed a blown reversing light bulb, and the low pressure in the nearside rear tyre was missed by 21%.
In a similar investigation conducted in 2007, Which? found 96% of garages failed to fix all the faults it set.
In the latest exercise, the watchdog also tested the garages' honesty by filling the car's screenwash and seeing whether garages charged for new fluid regardless.
Almost 40% billed for new screenwash.
Which? chief executive Peter Vicary-Smith said: "Almost all the garages in our investigation failed to fix basic faults.
"This could have endangered the lives of drivers and other road users and is simply not acceptable."
The watchdog is now calling for a new industry-wide qualification for all mechanics.
Which? found that garages affiliated to the Bosch Car Service and Motor Codes scheme performed better than average.
Garages endorsed by both codes spotted more than 60% of the faults.
But only 39% of problems were spotted by garages registered with the Good Garage Scheme.
Mr Vicary-Smith said: "The fact that the Bosch and Motor Codes schemes have out-performed independent garages is encouraging.
"But there's still a long way to go for the industry to win the confidence of car owners."
Rachel Greasby, from the Good Garage Scheme, said she was disappointed with the results.
She said: "Once we have further details on which garages are involved, a full investigation will be launched.
"Currently we carry out audits on all members bi-annually. The small minority who fail are removed from the scheme until they meet the required standards."
The Institute of the Motoring Industry, a professional association for people working in the motor industry, said its Automotive Technician Accreditation (ATA) scheme was working to improve the quality of car garages.
It said the scheme put garages through "rigorous assessment" to prove their competence, and in turn offered "reassurance" to customers.
David Moran from the RMI said: "Naturally we will work with members to improve upon the results and to ensure consumer confidence."
He said the body had already begun conducting its own investigations into "unacceptable garage services" and added: "[We] will be producing our own solution to this problem later this year, which will be aimed at benefiting the consumer and providing a safeguard against these sorts of issues."