|The Space Shuttle Discovery sits on Launch Pad 39B at the Kennedy Space Center at Cape Canaveral, Florida|
Last week, NASA halted Discovery's countdown less than three hours before launch when the sensor on the shuttle's hydrogen fuel tank malfunctioned.
Engineers have been conducting tests to find out why the sensor showed the tank running low when workers had filled it just a few hours before.
NASA says it may consider letting Discovery fly with only three of its four fuel sensors working normally.
The space shuttle program has been grounded since the shuttle Columbia broke up as it re-entered the Earth's atmosphere in February 2003.
NASA says the current launch window for Discovery could be extended until August 4.