Random police stops will now include a vehicle safety check and most drivers who fail will have 14 days to bring them up to standard.
The inspections will take up to one minute and be conducted alongside licence checks and breath tests.
Superintendent Carey Griffiths said the 3.5 million police stops each year are the perfect opportunity to check if vehicles are roadworthy.
The move is in line with changes to the warrant of fitness system, which will mean vehicle inspections are not required as often for newer cars.
Griffiths said motorists who fail to fix problems face a fine or their vehicle being taken off the road.
The Motor Trade Association says the roadside checks would not be needed if the warrant of fitness system had not been changed.
