NASA technicians found no damage after an electrical storm Friday afternoon, but said they will need at least another day to check critical systems.
A mission management team plans to meet early Sunday to determine if all the testing has been completed and whether to fuel Endeavour for a possible Sunday evening launch. Liftoff would be at 7:13 p.m. if NASA attempts to launch Sunday.
Mike Moses, chairman of that team, said there were 11 lightning strikes within three-tenths of a mile of the pad, although none of them struck the shuttle, external tank or the two solid rocket boosters.
At the pad, the shuttle has an elaborate lightning protection system with sensors and wires to direct lightning away from the shuttle and its rockets.
"The lightning protection system did its job," he said.
Moses said the assessment can be done in 24 hours, but there could be another delay if it isn't finished. A complete retest of the system could take two weeks.
Sunday's weather is expected to slightly better, with conditions 60 percent favorable for launch. The main concern will be thunderstorms.
Endeavour should have blasted off to the international space station in mid-June, but was grounded by potentially dangerous leaks of hydrogen gas. Repairs to a misaligned plate on the external fuel tank, which hooks up with a hydrogen vent line, solved the problem.
The shuttle and its crew are set to deliver and install the third and final piece of Japan's $1 billion space station lab, named Kibo — Japanese for hope. The first two sections flew up last year.
Seven shuttle astronauts plus six station residents will make for the biggest crowd ever in orbit.
Five spacewalks are planned during the 16-day flight.
NASA has until Tuesday or possibly Wednesday to send up Endeavour before making way for the launch of an unmanned Russian supply ship. After that, the shuttle flight would be off until late July.