NASA began the countdown for the launch of Endeavour after completing one final test to make sure the space shuttle's crew cabin was airtight.
"The team is ready. Endeavour is ready," NASA test director Stephen Payne said Sunday.
Last week, NASA replaced a leaky valve in Endeavour's cabin with one taken from the shuttle Atlantis. Engineers discovered that air had been escaping from the removed valve because of a small piece of debris on its seal, Payne said. The valve itself turned out to be fine.
Because of the extra work to replace and test the new valve, NASA delayed the flight by one day, to Wednesday.
Meteorologists are predicting a 70 percent chance of good weather for the early evening launch.
Endeavour's trip to the international space station for construction work features NASA's first educator-astronaut, Barbara Morgan. She was the backup for Christa McAuliffe, who died aboard Challenger in 1986.
Morgan will operate a robot arm in space and, if time permits, speak with school children at several locations across the country.
NASA hopes to stretch the mission from 11 days to 14 days, once Endeavour is in orbit, with help from a new system flying aboard the space shuttle. The system will convert and transfer power from the station to the shuttle, allowing the shuttle to remain docked longer than ever before.