A similar problem briefly delayed last summer's launch of Discovery on the first shuttle flight since the Columbia disaster in 2003.
NASA said it needs the time to open up the spacecraft's hydrogen fuel tank and replace the sensor, which gave an electrical resistance reading that was slightly off. The space agency plans to replace the three other sensors in the tank, too, to be safe.
The fuel gauges are designed to prevent the main engines from running too long or not long enough, during the climb to space. An engine shutdown at the wrong time could prove catastrophic, forcing the astronauts to attempt a risky emergency landing overseas, or leading to a ruptured engine.
"We wish it had worked out differently, but it's first and foremost that we fly safely," said Wayne Hale, NASA's shuttle program manager. "It was prudent to change these sensors out."
Despite the delay, Hale said he was still confident NASA will be able to launch three shuttles this year.
An investigation after last summer's sensor trouble showed that there may be a manufacturing problem that causes a loose connection between the 10-year-old sensors and wires, but further testing is needed to reach a conclusive answer, Hale said.
Replacing the sensors will take three weeks and require a worker to enter through the bottom of the 153-foot tank while it is upright.
The space agency had been working a tight schedule to meet the May launch date and had little room for any technical problems that might crop up.
In recent months, NASA has concentrated heavily on modifying the shuttle's big external fuel tank to prevent large pieces of foam insulation from breaking off during liftoff — the problem that doomed Columbia and its seven astronauts.
Last summer, despite 2 1/2 years of safety modifications and other steps to make the shuttle safer, large chunks of foam fell off Discovery at launch, to NASA's alarm.
The fuel tank sensor was not the only problem facing the space agency. Discovery's robotic arm was removed on Monday after a small crack was found in it over the weekend.
NASA managers had been debating whether to replace or repair the robotic arm. The launch delay now gives them time to ship the robotic arm back to Canada for repairs, but Hale said no decision had been made.
Technicians discovered the crack over the weekend using a visual magnifier and ultrasound equipment. The inspections were ordered after a work platform bumped the robotic arm more than a week ago while the workers were trying to clean up glass in the shuttle's payload bay.
The robotic arm has been used to inspect the outside of the shuttle with a camera, construct the international space station and release and retrieve satellites.