If rs.recordcount < 1 Then it seems not to be working it creates duplicates in symptomcode table.

Private Sub ErrorCode_AfterUpdate()
Dim db As DAO.Database
Set db = CurrentDb
Dim rs As DAO.Recordset
Dim strsql As String
strsql = "SELECT* from errorcode where symptomcode =" & Me.[ErrorCode] & "'"
Set rs = db.OpenRecordset("errorcode", dbOpenDynaset)
If rs.recordcount < 1 Then
Else
rs.AddNew
rs!symptomcode = Me.[ErrorCode]
rs.Update
End If
rs.Close
db.Close
Me.ErrorCode.Requery
End Sub