The only way I can duplicate your results is if the FindFirst does not
result in a match. Did you test rs.NoMatch after the FindFirst?

I'm guessing you will get a no-match anyway, because the left-most 13
characters of the string "Code" will always be the entire 4 characters of
that string.

Try two things: 1- remove the semi-colon ";" from the set statement and make
it just

Set rst = db.OpenRecordset("tblData")

2- I understand that "Code" is a variable, and you're searching for the first
13 bytes of that variable. If I understand correctly, you will want to change
your search command as follows:

rst.FindFirst "Left(Code, 13) = '" & Me.cmbLookup & "'"

I know it's hard to read on the screen, so I'll spell it out. First of all,
remove the quotes around the word 'Code' as it's a variable, not a literal.
Second, the FindFirst is looking for a SQL WHERE clause without the word
WHERE. Hence, give it SQL syntax, enclosed in quotes. After the equals sign
("="), put a single quote followed by a double-quote. The single quote
denotes the opening of a literal string (which will be provided by the "& Me.
cmbLookup" clause), and the double-quote to close the quotation and allow VBA
to solve for Me.cmbLookup (which cannot be done in a quotation). Follow the
'& Me.cmbLookup' with an ampersand followed by a space, a  double-quote, a
single-quote, and a double-quote (& "'"). The ampersands ("&"), of course, do
the concatenation of the string. The first double-quote opens the quotation
for the last part of the SQL statement, the single-quote closes the literal
string, and the last double-quote closes the entire SQL WHERE clause. See the
FindFirst method in the Help file for additional details.

HTH