Hi,
So you know that they re-prime themselves every 24 hours (turn off and back on 30 seconds later)
Mine sometimes wouldn't re-prime, and then I would have to take the motor off and make sure the impeller was not blocked, then put it back together and off it went.
This was rather annoying and on closer inspection I found that the impeller was actually broken. I'll try and explain the broken bit.
At the top of the impeller is the flat white disk that is the "blade" bit, and on top of this is a raised bit, which is kind of like a lip, (you could put an old 50c piece into it! Flat against the impeller blade) and I think the function of this part is to seat the impeller in the body of the filter.
On a cursory inspection it appeared that this part was fine, however if I gave it a bit of a wiggle I noticed that this part was 75% detached from the impeller blade. Once I replaced the impeller then I never had the issue again.
Theoretically you could take the motor off and just place the rest of the fx5 unit in your tank with air stones running in it to try and keep the bacteria alive, and just take the motor back to HFF if the impeller is fine.