Our car ran into some problems with overheating recently, but my partner is afraid of bringing it to the mechanic for a diagnosis of the problem, as he is afraid the mechanic will proclaim the car not road-worthy and he will not be able to use it until the problem can be fixed. He will be doing the night shift for the next couple of weeks and cannot rely on public transport to get to work.
My question is: Can a mechanic really legally proclaim a car to be not road-worthy and thus make it illegal for us to drive the car before the problem is fixed? Does he have the authority to do that? Or is my partner being paranoid (since he fully admits that he isn't positive but is just afraid of the possibility)?