Valve problem threatens to delay April shuttle launch, Spaceflgihtnow.com

"Engineers familiar with the system said the valve cannot be directly serviced at the launch pad. If the problem cannot be resolved by indirect methods or development of an acceptable flight rationale, the only option would be to roll Discovery back to the Vehicle Assembly Building, detach it from its external tank, haul it to its processing hangar and remove the OMS pod for repairs or replacement."