When the system boots it stops and displays reference code 551.

This code indicates that the rootvg will not varyon, and performing a maintenance boot allows the administrator to diagnose the issue and fix it. From the IBM AIX v7 Administrator Specialty Study Guide: "When the system is halted with a reference code 551, the rootvg will not varyon and the system will not boot. The administrator must perform a maintenance mode boot to diagnose the problem."