NASA engineers have been working one week to determine why a hydrogen fuel level sensor failed, halting the countdown for Discovery's flight to the International Space Station. The mission is poised to bring supplies to the station and, if successful, lead to the resumption of station construction, which has been stopped since the shuttle Columbia burned in orbit in 2003.

The problem sensor on Discovery gave a faulty low hydrogen fuel reading during a test just hours before the planned launch last week, even though the hydrogen fuel tank was full. An inaccurate reading could cause shuttle computers to command a premature shutdown of the engines during the climb to space.

The man overseeing shuttle launch preparations, John Muratore, says engineers have found a problem with the grounding of the wires to the sensor and apparent electrical interference from some equipment on the orbiter.

"We have worked our way through the system, starting with the sensor in the tank all the way back to the computers on the ground, and we have worked methodically to eliminate each part of that," he said.

Shuttle manager Bill Parsons says if engineers can restore proper sensor function within a couple of days, the launch countdown will begin again Saturday with the goal of a liftoff on Tuesday.

"We've done everything we can and we've eliminated the most probable cause. We believe the best way to go through this is to do a countdown. If the sensors work exactly like we think they will, then we'll launch on that day," he said.

If not, there will be more investigation and launch delay. Mr. Parsons says launch could take place as late as August 4. Otherwise, the mission will be delayed until September.

The current launch period and the one in September both allow daylight liftoffs when the space station is in the proper location to be reached most directly. NASA wants a daytime launch so cameras can record any debris falling from the external fuel tank like that which punctured Columbia during takeoff, dooming it to disintegrate.

When Discovery does make it to orbit, its astronauts are to conduct a spacewalk to test unprecedented methods to repair any surface damage a shuttle might incur upon launch.

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