Re,
Pour le principe si j'ai bien compris la question.
Public Sub TEST()
Dim ws As Worksheet
Dim I As Long
Application.ScreenUpdating = False
Set ws = ActiveSheet
With ws
For I = 340 To 357
If .Cells(I, "X") = "" Then
If .Cells(I, "W") = "PT" Then .Cells(I, "X") = "PLATEAU TECHNIQUE"
End If
Next I
End With
Set ws = Nothing
End Sub