Chosen Solution
When you reset Furby he just goes into sleep mode and will not wake up until he is reset again.
not sure if you have a furby with a tongue but…according to the Furby hospital this procedure may work. hold down tongue turn furby upside down shake furby a bit turn right side up and release tongue. You may have to hold tongue longer or try again. Just be gentle.
The only thing close to a motion detector circuit would be the Light sensor photocell in Furby’s forehead. I would start on that one and see if it is still functioning. For way more info on the Furby, check this site. It has great information and plenty of autopsy pics. Attached is the diagram for the Furby’s electronics. Hope this helps, good luck.
Do you mean the tilt sensor? Like the one that picks up if they’re swinging or like flying etc? (I have no idea how to find/fix them on eithet the old ones OR the newer ones, i’ll keep a lookout for you though…)