Q: I rented a car for a month, but it became unusable. Is the owner obligated to provide an alternate car?
A: This depends on whether you rented a specific car (zeh) or merely a certain model or size (stam). If you rented a specific car (e.g., from a private person or by license plate number), the owner has no further responsibility to provide an alternate car; you pay for the time that you used it (C.M. 310:1-2; 312:17).
However, if you rented a model or size and the car provided becomes unusable, the owner is responsible to provide an alternate one, even out of pocket. According to some authorities, the rental itself obligates him in this way; some maintain the owner is responsible only if there was also a kinyan sudar or signed contract (situmta); others say it is only if the owner wants his payment. There are many details involved (see Pischei Choshen, Sechirus 3:5-7.)
Of course, this issue depends on the stipulated terms of the rental and the local practice (Rema 314:2).