A day of trouble-shooting the problem with a faulty fuel sensor on the shuttle Discovery has not led to any definitive solutions, or a new schedule for flight.

NASA's Wayne Hale said at a Thursday news conference that "If we were to get extremely lucky" there could be a liftoff Sunday. But he admitted that's a highly optimistic "good luck scenario."

A "vast engineering team across the country" is trying to determine what caused an incorrect reading on the sensor Wednesday just hours before a planned flight. The sensor continued to malfunction after the tank was drained, reading "wet" for three hours more, Hale said.

After various test commands, the device worked properly for a time.

Hale did not rule out rolling the orbiter and its rockets and tank back to the vehicle assembly building.

---

Video: NASA Troubleshooting For Answers

Interactive: All About Space Shuttles

Video: Flight A Safety Check | Discovery Ready

---

The launch window for the Discovery closes on July 31. The next opportunity for a daylight launch, with the space station in close alignment, would be in September.

NASA is holding a countdown at 48 hours.

Hale said Wednesday if there's a quick, easy and safe fix, it would be possible to launch Discovery on Saturday. But if the part has to be replaced, Discovery won't lift off before Monday.

Mission Plan

The planned mission is to take a crew of seven to the international space station, where it will install a new gyroscope, among other tasks. They will also test methods for inspecting and repairing damage in flight.

There are three spacewalks planned during the mission.

Discovery is outfitted with a redesigned fuel tank and has dozens of motion and temperature sensors embedded in the wings to detect any blows from fuel-tank foam or other debris.