Tests on 114 Cornwall taxis led to about 10% being banned from use because of safety defects.
Police and Cornwall Council tested taxis in Saltash, St Austell, Camborne, Redruth, Bodmin, Hayle and St Ives.
Any unroadworthy vehicles have been prohibited from being used until the defects have been rectified.
The most dangerous faults included defective tyres, broken suspension and frayed seat belts. About 50% had minor defects like broken light covers.
Insp Martin Taylor said: "We will continue making these checks and hope people see this as a positive step to increase their confidence in the licensing and enforcement authorities."