CAPE CANAVERAL, Fla. (AP) – NASA scrubbed Friday’s launch of the space shuttle Atlantis again, this time because of a problem that has bedeviled the space agency before: a faulty fuel tank sensor.

The launch was rescheduled for 11:15 a.m. EDT on Saturday, when NASA will try a fourth time to get Atlantis off the ground and send it to the international space station to resume construction on the orbiting outpost for the first time since the Columbia tragedy 3 1/2 years ago.

Previous launch attempts over the past 12 days were dashed by a lightning bolt that struck the launch pad, Tropical Storm Ernesto and a fuel cell coolant pump that gave an erratic reading.

This time, a sensor in the hydrogen fuel tank gave an abnormal reading during a test as the shuttle was being fueled early Friday.

Atlantis’ six astronauts had donned their orange flightsuits and strapped themselves into the space shuttle, and the hatch to the shuttle had been closed, when NASA decided to postpone the launch with just 45 minutes to go until liftoff.

“We had a lot of discussion. … We follow the rules,” said launch director Mike Leinbach. “Ought to feel good that we did that.”

Atlantis commander Brent Jett responded, “We understand. We concur 100 percent.”

In order to fly, NASA would have had to waive a rule requiring that all four sensors work properly.

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.

Aboard Atlantis was one of the heaviest payloads ever launched into space _ a 17 1/2 ton truss section that will be added to the half-built space station. It includes two solar arrays that will produce electricity for the orbiting outpost.

Atlantis’ crew members will make three spacewalks during the 11-day flight to install the $372 million addition.

Construction on the space station has been on hold ever since Columbia disintegrated in February 2003. NASA spent the more than three years that followed trying to prevent the problem that doomed Columbia _ pieces of insulating foam breaking off from the shuttle’s big external fuel tank at liftoff.

NASA hopes this flight will jump-start an entire series of complicated shuttle missions which will culminate in 2010 with a completed space lab.