Liquid oxygen loading into the core stage was automatically halted near the beginning of slow fill operations when temperature readings on the propellant showed it was warmer than intended. The rocket is in a safe configuration while teams troubleshoot and determine a path forward.
Engineers believe they understand the issue and are working a solution that will allow operations to continue. Teams saw a similar issue during the wet dress rehearsal attempt on April 4, but at a slightly different point in propellant loading operations.
The slow fill process involves slowly filling the core stage with propellant to thermally condition the tank until temperature and pressure are stable before beginning fast fill operations, which is when you fill the tank at a quicker pump speed.