A problem with a low-level fuel cut-off sensor on an external fuel tank has caused the launch postponement.  These four sensors monitor the fuel levels in the tank during takeoff.  NASA is unsure what is causing the problem, so they are not speculating how soon the launch will take place.  At any rate, not today.

The low-level hyrogen sensor that is acting up seems to be reporting a low level of hydrogen, which of course is not accurate.  There are others of these sensors aboard that are working correctly, but NASA prefers a maximum state of "redundancy" for the launch, meaning even the backup systems have backup systems.  There is no danger to the crew, the shuttle, or the equipment--only a malfunction.

The crew is disembarking Discovery right now.

Spirits are low at NASA.  Troubleshooting is in progress.

Troubleshooting will occur today while Discovery is detanked.

This ship will launch on another day.

Over and out.