With the external tank one-third full of propellant, NASA crews are closely watching the engine cut-off sensors for any sign of failure.
Monitoring includes the sensors, the recently soldered pass-through connectors, even up through the flight deck, according to mission control.
The malfunctioning sensors were the cause for the December scrub of Atlantis.
NASA offficials sounded cautiously optimistic as the tanking has progressed past the point where the sensors failed in December.
One final test of the ECO sensors will be conducted at the T-9 minute hold before launch.
Weather still remains a concern with a 70 percent chance of weather prohibiting launch.