There are seven different welds on each engine duct. During prelaunch testing, the ground crew noticed an unusual response in the data transmission between the GPC and associated electronics hardware. The countdown had started and proceeded with little difficulty. Tanking was picked up using the backup pump 127 and the count proceeded normally. The four cores are all redundant allowing the Shuttle quad-redundancy.
Launch commit criteria rules require all four cores to be operating properly for safe flight.
The launch countdown was placed on hold at the T-minus 20 minute mark while commands were issued to determine if the problem was with the controller or with instrumentation . It was determined the problem was with the controller which will need to be replaced. They will cycle the hydraulic system's fill and dump valve and run a compressibility test. The hydrogen main fuel valve needed to be replaced which delayed the launch approximately one week. During the launch postponement press conference, Jim Harrington, KSC Launch Director and John Plowden, Rocketdyne Site Director reported that Tanking operations had begun approximately an hour later than planned primarily due to lightning in the area of the launch pad.