An electrical problem cropped up aboard Discovery again Tuesday after engineers thought they had it licked, and threatened to delay the shuttle's final liftoff just one day away.

The trouble appeared to be with a backup controller for one of the shuttle's three main engines.

The controller was sluggish early Tuesday morning, but shuttle officials found everything to be fine. Later, voltage irregularities were noted, prompting NASA to order last-minute reviews and hurriedly schedule a late afternoon meeting of top managers.

NASA spokeswoman Candrea Thomas said it was too soon to know whether Wednesday afternoon's planned launch would remain on track.

Discovery was supposed to blast off for the last time Monday on a trip to the International Space Station, but gas leaks forced a two-day delay.

NASA has until Sunday — possibly Monday at the latest — to launch the shuttle. Otherwise, the mission will be pushed into December because of unacceptable solar angles.

It's the next-to-last shuttle flight on NASA's official schedule as the agency looks toward newer and farther-flying craft. An extra mission may be added next year.