An investigation of used car sales and garages has revealed failings, according to inspectors.
Northumberland County Council Trading Standards officers posed as motorists taking their car for a service at five garages in the county.
Only one serviced the car to the manufacturer's specification and one charged for parts not supplied and work not carried out.
At 16 garages, 49 cars were inspected and four found to have major defects.
As well as the major defects, the inspectors said worn tyres were common.
'Bleak picture'
A vehicle examiner evaluated the condition of vehicles offered for sale and those who breached safety rules were advised to rectify faults before selling.
Four cars with major defects were immediately removed from sale.
Acting head of Trading Standards Jim Rutter said: "This survey painted a bleak picture which will give little comfort to motorists.
"It is clear there are shortcomings with some garages and dealers.
"It proved there is a need for continued consumer protection. We are planning more mystery shopping car servicing exercises and safety inspections of vehicles offered for sale or hire."