>>I have FB 2.5.3 on a laptop. Win 7 32 bit.

>>I introduced a stored procedure and in it call F_LEFT(:varName, 24) to truncate :varName from a vc(48).

F_LEFT belong to FreeUDFLib which you need to install.

Since you are already using FB2, it’s much easier to just use the built in LEFT() function instead:

http://www.firebirdsql.org/refdocs/langrefupd25-intfunc-left.html

-Maya