Conditional Appearance of msgbox

[Anshu]

Case:
There are 3 cells- "P20", "U20" and "Z20".
"Z20" contains the formula =IF(P20<U20,"??","")

I've this VBA in the sheet

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim xCell As Range, Rg As Range
    On Error Resume Next
    Set Rg = Application.Intersect(Target, Range("Z20"))
    If Not Rg Is Nothing Then
        For Each xCell In Rg
            If xCell.Value = "??" Then
                MsgBox "Entry Error"
                Exit Sub
            End If
        Next
    End If
End Sub

Private Sub Worksheet_selectionChange(ByVal Target As Range)
    Dim xCell As Range, Rg As Range
    On Error Resume Next
    Set Rg = Application.Intersect(Target, Range("Z20"))
    If Not Rg Is Nothing Then
        For Each xCell In Rg
            If xCell.Value = "??" Then
                MsgBox "Entry Error"
                Exit Sub
            End If
        Next
    End If
End Sub

Problem:
Msgbox appears only when I select the Cell "Z20" (If it contains "??")


My requirement:
1. Msgbox should appear immidiately after appearance of "??" in the cell "Z20"
That is, whenever a value is entered in the cell "P20" or "U20", and P20<U20 (as the formula set in cell Z20), immidiately the msgbox should appear.
2. Also, the msgbox should appear when the cell Z20 contains "??" and the cell Z20 is selected. (It is working currently)

Please find the attachment below for more clarity.

Many Thanks!