Nasa will go ahead with the rescheduled launch at 10.39am (1439 GMT) on Tuesday if the problem does not recur or if it is only found in the two sensors that have been rewired, Wayne Hale, deputy manager of the shuttle programme, said at a news conference late on Sunday.
"If the problem recurs ... we're going to do some more tests just to make sure we understand what is causing this to happen. And if we're comfortable that we have a good understanding, then we can go fly," Hale said.
Workers at Nasa have switched the wiring between the problem sensor and another one after the gauge failure forced the space agency to postpone the space shuttle's launch while astronauts were boarding Discovery on 13 July.
Nasa's own launch rule, in place since the 1986 Challenger disaster, requires that all four hydrogen fuel gauges in the external tank be working properly.
Nasa Administrator Michael Griffin said he was comfortable with the decision and even hoped the problem would recur to help further pinpoint the source of the trouble.
He acknowledged the public might think the space agency was rushing to launch, but insisted it was the right technical judgment.
"It's not a safety-of-flight issue," Griffin said.
Although the focus of Nasa's attention has been on the sensor, rain and clouds may end up causing more concern on launch day.
Forecasters put the odds of good launch weather on Tuesday at 60%. The weather at the overseas emergency landing sites also was not looking good.
Nasa has just one week to launch Discovery and its crew of seven to the international space station before it would put off the mission until September.