§1. When a see is impeded, the coadjutor bishop, if there is one, has governance of the diocese unless the Holy See has provided otherwise. If there is none or he is impeded, governance passes to an auxiliary bishop, the vicar general, an episcopal vicar, or another priest, following the order of persons established in the list which the diocesan bishop is to draw up as soon as possible after taking possession of the diocese. The list, which must be communicated to the metropolitan, is to be renewed at least every three years and preserved in secret by the chancellor.
§2. If there is no coadjutor bishop or he is impeded and the list mentioned in §1 is not available, it is for the college of consultors to select a priest to govern the diocese.
§3. The one who has assumed the governance of a diocese according to the norm of §§1 or 2 is to advise the Holy See as soon as possible of the impeded see and the function he has assumed.
Source: Code of Canon Law (Vatican.va)