Discovery's six astronauts planned to spend their last full day in space on Sunday stowing equipment, giving television interviews and testing the space shuttle's flight control system for landing.
The testing relieved concerns about a leaking unit that powers hydraulic systems in steering and braking.
Engineers were not sure if it had been leaking harmless nitrogen or flammable hydrazine and had considered burning off the fuel and shutting down the unit before landing to eliminate any fire hazard - something NASA has never done before. The spacecraft needs only one power unit to land.
Without a third power unit, Discovery's landing gear would have to be deployed by explosive charges rather than the hydraulics system and stricter weather requirements would be implemented for the shuttle's landing at the Kennedy Space Centre in the US state of Florida.
But early testing results on the unit this morning showed no problems.
"We saw normal fuel usage, normal parameters," Mission Control radioed Steve Lindsey, Discovery's commander.
Lindsey responded, "OK. Great news."
NASA managers already were keeping an eye on the weather as thunderstorms Monday morning threatened to be within 30 miles of the Kennedy Space Centre's 15,000-foot runway.
"It looks generally pretty good but they are showing a chance of showers," Mission Control told Lindsey. "It gets a little worse as the day goes on so we're hoping the early morning works out for us.