The sensors ensure the shuttle's main engines shut down before they drain the liquid oxygen and liquid hydrogen propellants from the fuel tank.

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.

The US shuttle fleet, which is scheduled for retirement in 2010, has been grounded since last July, when insulation foam broke off the external fuel tank of the shuttle Discovery as it was launched from Cape Canaveral in Florida.

Falling insulation foam was the same problem that doomed the shuttle Columbia in 2003.

The next opportunity to launch Discovery to the International Space Station will be between July 1 and 19.

Despite the delay, NASA shuttle program manager Wayne Hale said he is optimistic NASA will be able to fly three shuttle missions this year.

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. Enditem

(Agencies)