Taxis were checked as part of a scheme to drive up standards in the city.
Licensing bosses at Preston Council teamed up with the Vehicle and Operator Services Agency (VOSA) to invite more than 20 taxis in to be tested.
They were inspecting taxis which had recently been handed improvement notices on their MOTs or where faults had been found during compliance checks.
During one morning where 10 vehicles were checked, four drivers were handed immediate prohibition notices and two were handed delayed prohibition notices.
Senior licensing technician Philip Walsh said: “I think drivers are beginning to accept that cabs have got to come up to high standards.
“Really, we expect everything with the taxis to be spot on because they have been invited to us, but they are not. Things have got to change.
“We always give praise where it is due but we are not afraid to tell them when there is something wrong.
“The drivers do appreciate it and the vast majority don’t mind being told when there is a fault. If taxis are regularly maintained they won’t have these problems.”
Examiners were looking at bodywork and tyres on taxis, and well as things like ramps and seatbelts.
Mr Walsh said: “We come across all sorts. One thing we have come across is tinted film on windows, which is not allowed.
“We are looking at things like body work and corrosion.
“It is also a check on our test centres. We are trying to improve standards all the time.
“The public are paying good money and they want a nice clean cab and a driver who takes pride in what he is doing.”
Coun John Swindells, cabinet member for regulation, said: “The council needs to be satisfied that licensed vehicles operating within Preston are safe to do so.
“Each licence is for a period of only six months and in order for it to be renewed, each vehicle is examined and tested at a council approved garage.
“In addition to the six monthly examinations the council undertakes regular enforcement operations with its partners, VOSA and the Road Police Unit.”