NASA (search) officials began a second fueling test Friday on space shuttle Discovery (search) in an effort to understand why sensors and valves weren't working properly during a previous test.

During the test, part of the preparations for a planned launch in July, the external tank will be filled with 500,000 gallons of liquid hydrogen and liquid oxygen.

"Everything's going fine," said NASA spokesman Bruce Buckingham after the test started.

Discovery's first fueling test last month uncovered sensor and valve problems that still puzzle engineers.

The second test comes just days before Discovery will be rolled back into a hangar to replace its tank with a safer, updated model. Shuttle managers decided to remove Discovery's fuel tank, which is attached to a pair of booster rockets, and install a brand new set that had been meant for the second post-Columbia flight, by Atlantis.

A heater will be inserted on the new tank to prevent the buildup of ice once super-cold fuel is pumped in right before liftoff. Engineering tests found ice to be as dangerous as flying foam. As a result, the launch date was bumped from late May to mid-July.

A large chunk of foam insulation broke off Columbia's fuel tank during launch and gouged a hole in the left wing, dooming the spacecraft and its crew during re-entry on Feb. 1, 2003. All seven astronauts were killed.

Also Friday, NASA's new administrator, Michael Griffin, will meet with workers at the Kennedy Space Center (search).