CAPE CANAVERAL, Fla. (AP) – 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 21/2 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.”

A liquid hydrogen pressurization relief valve that opened and closed more times than normal during last month’s test may have been partially caused by a change in the manufacture of a mesh screen at the top of the hydrogen tank, Parsons said.

The valve opens and closes to ensure the liquid hydrogen stays at the correct temperature. The screen disperses the hydrogen fuel as it goes in the tank to prevent hot spots.

Discovery will fly using an old-style screen.

NASA Administrator Michael Griffin, who took NASA’s top job last month and was visiting the Kennedy Space Center Friday, said he did not believe there were any major hurdles to returning the shuttle to flight.

“I’ve seen just the normal bumps in the road that you have to get past, especially when you haven’t flown in two years and several months,” he said.

The second test came just days before Discovery will be rolled back into the hangar to replace its tank with a safer, updated model. Also, a heater will be installed on the new tank to prevent the buildup of ice once the super-cold fuel is pumped in.

Engineering tests found that ice falling off the tank could be as dangerous as the chunk of foam insulation that doomed Columbia.

The danger of ice and the sensor-and-valve problems prompted NASA to postpone Discovery’s launch from late May to mid-July.

The board that investigated the Columbia accident criticized NASA for stifling dissent. Griffin said the decision to postpone Discovery’s launch shows that the space agency is changing its culture and becoming more open to different opinions.

“Everybody wanted to fly. I wanted to fly,” Griffin said. “But the team made the right choice.”

A large chunk of foam insulation broke off Columbia’s fuel tank during launch and gashed the left wing, dooming the spacecraft and its crew during re-entry in 2003. All seven astronauts were killed.