Not sure where I posted this before, may be inaccurate anyhow
https://www.mobileread.com/forums/att...6&d=1434748263
Any dirt/obstruction/misalignment in the way and touch is dead.
Of course if one of the emitter/sensors is dead there is nothing you can do anyhow...
With strong IR emitters (like the Wii sensor bar) you can verify the functionality with a digital camera but that's not possible for IR touchscreens... (or at least, I have not managed to).