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.
Mr 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.
Newer cars are required to have less frequent safety inspections, and the association said this allows more time for vehicles to develop problems.
Association spokesperson Ian Stronach said roadside checks done by police would be less thorough than those completed by professional mechanics.
"How effective will roadside checks be. So can they check everything as well as someone in a workshop - probably not. How many are they going to get done and potentially will that lead to queues on the side of the road with cars being checked."
Mr Stronach said the inspections mean extra work for an already stretched police force.
But the Automobile Association says it fully supports police doing tests on vehicles.
General manager of motoring affairs Mike Noon said with longer periods between inspections for many cars, drivers are responsible for keeping their vehicles roadworthy and the association strongly supports any initiatives making driving safer.