Sorry to say, no STAR for your reputation - - -yet
seemed like good logic
well explained
BUT
No error, and no change in behavior
I tried it in Afterupdate , dirty, and on the 2nd combobox ONCLICK
I think I must have a typo bug - - - this makes no sense
I also tried using the correct box name:
If Me.COMBOcatTP.OldValue <> Me.COMBOcatTP Then
Me.COMBOsubC = Null
End If
COMBOsubC.Requery