Have you heard the term ECO sensor? If you care about space shuttle launches, you probably have. These are the Engine CutOff sensors. They are in the external tank and should prevent the main engines to run dry in a failure mode (that could lead to vehicle destruction). In essence, they simply indicate if the tank is empty or not (much like the tank gauge in your car, but "a bit" more high tech).
The ECO sensors are known problem sources and are responsible for more than one launch scrub. In recent launches, however, they worked quite well. So we hope that this won't be a problem for Atlantis STS-122 launch. If there is a problem, it would be detected some time around now, during tanking. Then, test commands are sent to check all sensors out.
An additional problem with the ECO sensors is that if they are faulty, they can not be quickly accessed for repair. The reason is that that they in the external tank, not accessible. Thus, as far as I know, there was always a waiver to launch with the problem, if there were one. However, I think this is possible only if there is at most one sensor failing.
I have now also written an in-depth article about the space shuttle's ECO sensors. If you like some details and cool facts, I suggest reading it ;)
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