Could you imagine what the doctors would say if you had a backup heart?
Go ahead and climb that moutain, if #1 acts up...
Some good articles on the sensors.
From http://www.spaceflightnow.com/shuttle/sts114/050724postmmt/
Here's the ECO sensor launch strategy at a glance:
Any ECO sensors "fail dry:" Scrub
More than one hydrogen ECO sensor malfunctions: Scrub
Any liquid oxygen ECO sensors fail: Scrub
Hydrogen ECO sensors 1 or 3 malfunction: Scrub
All four hydrogen ECO sensors work normally: Launch
Hydrogen ECO sensor 2 fails wet: Launch
Hydrogen ECO sensor 4 fails wet: Launch
Sensor info; http://www.spaceflightnow.com/shuttle/sts114/050716ecographic/
Twenty four propellant sensors are used in the shuttle's external tank, 12 each in the oxygen and hydrogen sections. Eight are used in each tank to measure the amount of propellant present before launch. Four in each tank, known as engine cutoff - ECO - sensors, are part of a backup system intended to make sure the ship's engines don't shut down too early, resulting in an abort, or run too long, draining the tank dry with potentially catastrophic results.
NASA's original launch commit criteria required three operational ECO sensors for a countdown to proceed. But in the wake of the 1986 Challenger disaster, the LCC was amended to four-of-four because of concerns two sensors could be knocked out by a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. The single-point failure was corrected during Discovery's last overhaul, but the four-of-four launch rule remains on the books.