CAPE CANAVERAL, Fla. -- NASA conducted a second fueling test Friday on space shuttle Discovery to try to figure out why sensors and a valve did not work properly during a previous run-through.
The test was in preparation for the launch of Discovery in July on the first shuttle flight since the Columbia disaster nearly 2 years ago.

Discovery's external tank was filled with 500,000 gallons of liquid hydrogen and oxygen, and a countdown was simulated.

Everything went without a hitch this time, but NASA engineers evaluating the preliminary data were still unsure why the liquid hydrogen sensors gave intermittent readings during last month's test. The sensors act as fuel gauges that notify the shuttle's main engines to shut down when propellants reach a certain level.

"We didn't find any smoking gun," said Bill Parsons, manager of the space shuttle program.

"At this point, the conclusion that you might come to is that we had some kind of connection that wasn't exactly right ... and therefore we've cleared this up."