The space agency has been conducting tests and believes the fuel gauge problem is intermittent.
"Personally, I think that we've done an extensive degree of troubleshooting and analysis to best understand what we've got," said Pete Nickolenko, a NASA test director.
NASA may approve a waiver if one of the four gauges located at the bottom of the external fuel tank doesn't work during refueling. Such waivers are rare, Nickolenko said. He expressed confidence the sensor would work properly.
Only two of the four gauges are needed to ensure safety, but ever since the 1986 Challenger explosion during liftoff, NASA has required all four gauges to be working.
The fuel sensors are needed to prevent the main engines from shutting down too soon or too late during liftoff. The first scenario could result in a risky, never-attempted emergency landing; the second could cause the engine turbines to rupture and destroy the spacecraft, reported the Associated Press.
Engineers are still uncertain why the sensor failed to work on July 13, causing NASA to postpone Discovery's 12-day mission to the International Space Station.
Tuesday's launch is set for 10:39 a.m. The launch window remains open until the end of the month. The next launch window opens in September.
Weather may cause NASA to postpone liftoff. Forecasters have put the chances of good weather on Tuesday at the shuttle's launch site at Cape Canaveral, Fla. at 60 percent.
When Discovery takes off, it will be the first shuttle flight since the Feb. 1, 2003 Columbia disaster. A suitcase-size piece of foam that had broken off during launch and punched a hole in Columbia's heat shield allowed superheated gases to penetrate the wing and caused the orbiter to break apart during re-entry, killing all seven astronauts aboard.
Since then, NASA has been working on various safety mechanisms and practices, such as launching the shuttle during daylight so it can better photograph the orbiter in case of a potential problem, installing heaters to prevent ice on the fuel tank, and developing new techniques to repair the heat-resistant tiles while in orbit.
Discovery flew its first mission on Aug. 30, 1984. The upcoming mission will be its 31st, according to NASA.