Execute same routine for multiple rows

How about

Sub AutoPart()

Dim Rng As Range
Dim X As Range

   Set Rng = Application.InputBox(Prompt:="Select a cell.", Title:="X-Axis.", Type:=8)
   For Each X In Rng
      If X.Offset(0, 12).Value Then 'If a value is present in column M Then
          Range(Cells(X.Row, 34).Address) = "P/N Proof" 'insert "P/N Proof" in column AG
      End If
      
      If X.Offset(0, 13).Value Then
          Range(Cells(X.Row, 34).Address) = "P/N Static Pressure"
      End If
      
      If X.Offset(0, 14).Value Then 'If a value is present in column O Then
          Select Case X.Offset(0, 1).Value 'Look in column B for one of the following cases:
              Case Is = "XYZ"
                  Range(Cells(X.Row, 34).Address) = "P/N XYZ" 'If Case XYZ insert "P/N XYZ" in column AG
              Case Is = "ZXY"
                  Range(Cells(X.Row, 34).Address) = "P/N ZXY"
              Case Is = "YXZ"
                  Range(Cells(X.Row, 34).Address) = "P/N YXZ"
              End Select
      End If
      If X.Offset(0, 15).Value Then
          Select Case X.Offset(0, 1).Value
              Case Is = "123"
                  Range(Cells(X.Row, 34).Address) = "P/N 123"
              Case Is = "321"
                  Range(Cells(X.Row, 34).Address) = "P/N 321"
              End Select
      End If
      If X.Offset(0, 16).Value Then
          Select Case X.Offset(0, 1).Value
              Case Is = "456"
                  Range(Cells(X.Row, 34).Address) = "P/N 456"
              Case Is = "654"
                  Range(Cells(X.Row, 34).Address) = "P/N 654"
              Case Is = "546"
                  Range(Cells(X.Row, 34).Address) = "P/N 546"
              Case Is = "564"
                  Range(Cells(X.Row, 34).Address) = "P/N 564"
              End Select
      End If
   Next X
End Sub

Brilliant! It looks like you created a loop so that each X runs the routine within the range set by the input box. Thank you so much.

Don't worry about the "Instr" problem. I found that it works if I scratch the "select case" method and opt for the "if then elseif" statement

      If X.Offset(0, 15).Value Then
        If InStr(1, X.Offset(0, 1).Value, "123") Then
            Range(Cells(X.Row, 34).Address) = "P/N 123"
        ElseIf InStr(1, X.Offset(0, 1).Value, "321") Then
            Range(Cells(X.Row, 34).Address) = "P/N 321"
        End If

Glad to help & thanks for the feedback.