ELACs are equivallent computers (also applies to SEC computers) which means that if you get ELAC 2 fault before departure, the best and quickest solution is to swap the computers (ELAC 1 inop and ELAC 2 ok).
Guess ain't that easy sir. It is important, when swapping or replacing computers, to strictly adhere to the mixability and interchangeability rules laid out in service bulletins and IPC. Two ELACS thought to be analogous, might belong to different standards though, thus leading to unexpected system operations.
fredgrav