I do a lot of work on Jungheinrichs and one of my customer despite telling them on several occasions be careful when jet washing (only if you have to), they still go mad on it. On a few occasions we have had this zero lift sensor error on the EFG 16ac, and what we found is the interface card that sits at the front on the right hand side under the floor plate gets wet and corrosion on the pcb, sometime leaving it to dry solves the issue other times we need to dry the card out and resolder the connections.

As danny said worse case you will end up forking out serious money for these controllers.