Jungheinrich TFG316s

Sideshift fitted but no operation, used to work fine but now nothing. Haven't investigated yet but looking for pointers if possible. Fingertip control, no errors on display and all other hyd functions work fine. Operating 3rd lever does produce engine note change. Seems like an electronic valve issue to me, which is where i would probably start off, there's also a 4th line fitted but with quick release couplings on the end. These outputs are not in use. No space for a 4th lever though. Can anyone shed any light on this at all?