The fuel-sensor gauges are used as backup devices for timers and other sensors that are the primary way for the mission computer to know when to shut down the Space Shuttle Main Engines (SSMEs).

In the case of the faulty sensors for the hydrogen tank, the sensors indicate the level of liquid hydrogen. They sense a “wet” condition when the tanks are full, but a “dry” condition when it is time to shut down the engines because the hydrogen fuel is almost gone.

NASA does not know—going into the test—whether they will be able to find the problem (thus, allowing a quick turnaround for the launch of STS-122) or not be able to find the problem (thus, forcing them to delay further the next mission in order to fix the problem).

NASA will be looking within the approximate 100 feet (30 meters) of wiring that run between the four fuel-sensor gauges located at the bottom of the fuel tank. A tool to diagnosis the problem, what NASA calls a time-domain reflectometer (TDR), will be used to (hopefully) find the faulty point in the circuit that is causing all of the problems.

The TDR will be inserted into the wiring that is involved with the sensor problem. When activated, the TDR will use timed pulses to sense any defects in the circuit. The amount of time that the timed pulses travel to the TDR will indicate the approximate location of the fault.

Other NASA engineers will be off-site looking into other avenues that may be the culprit in the malfunctioning sensors.

Shuttle program manager Wayne Hale is quoted in the December 11, 2007 MSNBC article “NASA to test shuttle’s glitchy fuel tank,”: "We're going to go where the data leads us to go.”and "Our point is to try to fix this system today."

Hale went on to say that the earliest time for a launch is January 2, 2008, but added that it is a great possibility that the launch date will be later than that; specifically saying it “could definitely be a little bit later than that.”

However, there might be more to the story .....