So I just bought this used guitar: "Seagull S6 Coastline Slim CW Spruce QI", which was in excellent condition except for 
the piezo that does not provide sound for the high "E" string.

I'm not able to find the Godin QI system for sale anywhere (or Quantum QI, or QIT, whatever the name). I'm also not sure 
if this kind of problem is due to the piezo or the preamp. And I absolutely can't find if it's possible to change just the 
piezo and keep the electronics, if the  piezo is the faulted part.

If I can change just the piezo, is there different kind of qualities, because all I seems to find is really cheap ones. 
Finally, Does the piezo makes the quality of the sound, or is it the preamp?

BTW, the guitar sounds great and the electronic part is not very important to me, but if I can fix this, I'd rather do it.

Thanks a lot! I must add that I searched on the net but could not find anything releveant on the QIT system.
Undersaddle piezo pickups work because the vibrating string changes the pressure on the saddle and the saddle passes the changing pressure to the piezo.  If only 1 string is quieter it typically means you don't have good contact between the piezo strip and the saddle meaning the piezo can't sense the changes in pressure.  Your problem is most likely the result of your saddle not being seated well enough inside the bridge but could also be caused if the saddle isn't completely flat on the bottom, the channel routed into your bridge could be uneven, or the piezo strip under the saddle might not be positioned correctly.  I suggest you pull the saddle out and make sure the piezo strip is flat and runs the full length of the channel in your bridge. Then reinstall the saddle.  If the problem persists you might need to sand a little off the bottom of your saddle to make sure it's 100% flat. 
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Last edited by CorduroyEW at Apr 18, 2017,
Thanks a lot CorduroyEW !

I'll check that tonight. I'd prefer not to sand the bottom of the saddle, because the action on the guitar is already very low, and I'm afraid it will lead to some string buzz. But if it appears to be the solution, is there an alternate way of doing it, by adding a very slice piece of wood between the piezo and the saddle? Or under the piezo? Do you think, this could also be a solution?

thanks again.
If you sand the saddle and the action is too low afterward then a hardwood shim can be used to bring the action back up again.  If you use a shim then put it under the pickup.  Chances are the saddle simply needs to be seated better and the act of taking it out and putting it back in again will fix it but if you do have to sand the saddle down a bit you probably won't need to take enough off to make a noticeable change in the action.  To insure you take as little off as possible I would suggest scribbling all over the bottom of the saddle with a pencil or felt pen make 1 pass at a time over your sandpaper.  After each pass look at the bottom of the saddle and you will see where material is being removed.  Once the graphite (or ink) is being removed from the entire length equally you have sanded far enough.  Also make sure you read a tutorial or two about the proper way to sand down your saddle.  The sandpaper has to be on a flat hard surface, your saddle needs to be at a perfect 90 degree angle to the sandpaper and you should sand in 1 direction only.  If you sand using a back and forth motion, the saddle isn't at a 90 degree angle to the paper, or the paper isn't on a completely flat surface then you are going to make your problems worse not better.  
Not taking any online orders.
@  CorduroyEW

All right then. I have to change the strings, so I'll remove the saddle and put it back in, and then we'll see and hope this works without having anything else to do. Thanks for your time! It's really appreciated.
In addition to Corduroy's good suggestions you could try:

1) A small shim for the high E string under the saddle.
2) Cutting vertical slots in the saddle between the strings ala Fishbone saddle to make it more flexible.
2) Cut ramps between the bridge pin holes and the saddle to increase the string break angle over the saddle and hence greater downward pressure.

I have successfully used a combination of these where levelling didn't do the trick, maybe due to a a weak piezo element.

It is possible that the piezo has died under the high E string, this happens sometimes, and the whole UST has to be replaced.
Thanks for the suggestions Tony Done, but it seems that simply changing the strings, checking if ths piezo was properly seated and reinstalling the saddle did the trick for me. I must say that the strings were totally rusted, so maybe the previous owner didn't  really used it (that's what he said) or didn't took enough care of the guitar (but he said he took good care of it). I didn't really saw how much the strings were bad, untill I removed them and the difference in color became obvious where the strings are hidden by the pegs. But the guitar is now like new and everything works beautifully. 

thanks everyone for the help!