NASA has delayed its first space shuttle launch of the year for a fourth time. No new date has been set.
The agency has concerns about a potential problem with fuel pressurization valves.
Discovery was scheduled to take off on 27 February for a 14 day flight to outfit the International Space Station with a final set of solar wing panels.
The flight has already been delayed three times.
A tiny piece of a valve for an external fuel tank cracked during the shuttle's climb into orbit on the last mission in November.
NASA says there was no danger to the shuttle or its crew.
Nine shuttle flights remain for construction of the station to be completed, and a final servicing call made to the Hubble space telescope. The fleet is scheduled to be retired in 2010.