how to avoid infinite loops ine excel vba ?

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Option Explicit

Sub IbanNo()
    Dim LR As Long
    Dim iX As Long
    Dim rRng As Range
    LR = ActiveSheet.UsedRange.Rows.Count

    For iX = 4 To LR

        If Not IsEmpty(Cells(iX, 16)) And Left(Cells(iX, 16), 4) <> "PT50" Then
            If rRng Is Nothing Then
                Set rRng = Cells(iX, 16)
            Else: Set rRng = Union(rRng, Cells(iX, 16))
            End If
        End If
    Next iX
    With rRng
        .Interior.ColorIndex = 3
    End With
End Sub