What NASA is saying is that they don't know that it really is just one sensor -- that maybe it's a system error of some kind and this is just the first symptom. The idea is that if they don't know what caused it they won't be certain all the sensors won't go out. My personal feeling is that four is overkill and just gives more opportunity for something to go wrong. Also, I have a hunch that the real problem is inside the tank, something NASA doesn't want to face because it would require very extensive, time-consuming repairs. They pointed out that the more they get inside various components and change things out at this point, the greater the chance that they will break something -- maybe some critical element whose damage won't show up until it's too late. I would be surprised if they do make the July launch window. -- Joe