10:38 PDT -- a low level (cutoff) external fuel tank sensor is the culprit. (transcription summary of NASA TV commentators:) The ECO measures the amount of hydrogen remaining in the tank; when triggered, it helps the systems ramp down the engines as they approach the end of remaining fuel. There are four such sensors; two must work. With an anomolous reading from one sensor, the flight bosses are using "conservative engineering." There probably will be no launch today.
11:10 PDT -- the crew motorhome is travelling back to the Vehicle Assembly Building right now. A press conference will occur no earlier than 1:30 PDT; no estimate will be made as to the duration of the delay until they have finished their troubleshooting of the troubled ECO sensor, which has just gotten underway.
3:00 PDT -- the earliest new launch date will be Saturday, July 16.
5:12 PDT -- the culprit is apparently a guage that showed an erroneous "full" reading when it was sent test data that should have resulted in an "empty" reading. The other three ECO devices registered properly. This is akin to the gas guage on a car always reading full (although the Shuttle has four of them). The ultimate problem is not that the Shuttle will run out of fuel, since there is no Chervron station to pull into, but that the engines would not properly shutdown as they approach the end of their fuel supply, which could lead to a catestrophic failure of an engine (i.e., kaboom). So, reload for Saturday.
Incidentally, here is a picture of the launch site (the pad in use this time is in the upper right of the image; the Vehicle Assembly Building is the complex in the lower left of the picture):
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