Try this. I think your trying to substitute a comma in a number which needs
an extra step. This works for text and numbers

=SUBSTITUTE(TEXT(A1,"0,000"), ",","_")

"BorisS" wrote:

how do I write in a substitution of a comma? SUBSTITUTE does not seem to
like it when I just type SUBSTITUTE(reference, ",","_"). I need it
substituted with an underscore.

Thanks.
--
Boris