Safety checks on more than 100 child car seats in Powys found that nearly all of them had problems with the way they were fitted, according the council.
Out of 115 seats checked in Brecon, Builth Wells, Machynlleth, Newtown and Ystradgynlais, 92 were found to have problems.
Council road safety unit adviser Fran Vickery carried out the survey.
Powys Council said it was disappointed more parents did not take part.
Fifty seven per cent of the seats examined had a minor problem that needed to be rectified, which included tightening the seatbelt, said the council.
It said 12% of the seats had major faults, which included the seatbelt being incorrectly routed, while 6% were unsuitable for the vehicle or the child and should not have been used.
Councillor Geraint Hopkins, who is responsible for road safety, said: "I'm pleased that these checks meant that so many of the faults identified were rectified.
"However, I was disappointed that more parents did not support this wonderful project.
"We go out of out way to ensure that children are travelling around Powys as safe as possible and it is disheartening that some parents will not allow a few minutes out of their say to look after their child's safety in the car."