Sub test()
OldTxt = "oui" 'pour le test
NewTxt = "NON" 'pour le test
With Sheets(I).Range("F12:J211") 'ici tu ne traite que la plage "F12:J211"
.Replace What:=OldTxt, Replacement:=NewTxt, LookAt:= _
xlWhole, SearchOrder:=xlByRows, MatchCase:=False
End With
End Sub