If the Server Has Normal Redundancy

If you are using normal redundancy, then the server has one mirror copy. The data could be irrecoverably lost, if that single mirror also fails during the rescue procedure.

Oracle recommends that you duplicate the mirror copy:

Make a complete backup of the data in the mirror copy.
Take the mirror copy server offline immediately, to prevent any new data changes to it before attempting a rescue.

This procedure ensures that all data residing on the grid disks on the failed server and its mirror copy is inaccessible during the rescue procedure.

The Oracle ASM disk repair timer has a default repair time of 3.6 hours. If you know that you cannot perform the rescue procedure within that time frame, then use the Oracle ASM rebalance procedure to rebalance the disks until you can do the rescue procedure.