Florida ? With a launch countdown looming, NASA scrambled Wednesday to fix a cabin leak aboard space shuttle Endeavour.

The leak was traced overnight to one of two pressure-relief valves in Endeavour’s crew cabin, located behind the toilet but separate from the bathroom plumbing, said NASA spokesman George Diller.

NASA decided late Wednesday afternoon to replace the bad valve with one taken from space shuttle Atlantis. The newly installed component will be tested today.

A problem with one of two thermostats for one of Endeavour’s auxiliary power units also cropped up. Engineers initially thought both thermostats would need to be replaced, but they concluded that the faulty one is good enough to fly.

All the extra work can be completed in time for Endeavour’s planned liftoff Tuesday, Diller said, but it will be tight and anything else – like more thunderstorms or mechanical problems – could force NASA to postpone the flight to the international space station.