A primary I/O adapter and a redundant I/O adapter of a data processing system are assigned to support access to a system resource. While the primary I/O adapter is in service and the redundant I/O adapter is not in service in providing access to the system resource, a fail over command is issued to remove the primary I/O adapter from service and place the redundant I/O adapter in service in supporting access to the system resource. While the redundant I/O adapter is in service and the primary I/O adapter is not in service in providing access to the system resource, diagnostic testing on the primary I/O adapter is performed. In response to the diagnostic testing revealing no fault in the primary I/O adapter, a fail back command is issued to restore the primary I/O adapter to service and to remove the redundant I/O adapter from service.